Parakeets can be fairly messy and while they don't rival the larger parrots noise wise, they are somewhat noisy. Also, they should be kept in at least pairs.
I do not think they are ideal pets for young children or for someone who wants a bird that they can really handle. Unlike larger birds, parakeets require a LOT of work to become extremely tame. Mine will step up, but they dislike being petted, dont want their necks scratched etc. The extent of our interaction is that they will sit on my finger. And they don't even do that for long. I grew up in a household full of different parrots, and to get a really interactive pet, you want a larger bird. A cockatiel would probably be the most ideal if you don't have the experience or commitment for a larger parrot.
They also need large cages- the ones you see in pet stores are almost all far too small. My current cage is 30inches X 18inches X 18 inches for a pair. You wouldn't go much smaller for a single bird.
They also need a lot of toys, which you rotate in and out of the cage. Many bird toys are designed to be chewed up and destroyed and they can be expensive. Its not fair to only buy the non-destructible ones because most birds have a natural urge to chew. The same with perches- they need a variety of sizes and textures and they should also be rotated. If they only sit on the generic round dowel perches they can develop foot issues.
Another issue with parakeets and all birds is they need a varied diet. All seed diets can cause issues for birds. Many birds are smart and learn which seeds are their favorites, and then pick them out and only eat those. A diet of just one or two seed types is obviously not balanced, but you can't force a bird to eat the other seeds if he doesn't want to. You can feed pellets, but they also aren't ideal for parakeets, and since they're designed to meet the needs are many different species, may not be completely balanced. For budgies (parakeets) a mix of seeds and pellets seems best- but getting them to switch to pellets or trying any new food is a challenge! They should also get some other foods- fresh veggies, fruits, breads etc. Its important for their health, but you have to work really hard to get them to eat it, sometimes it takes months of giving them the new food daily before they will try it and accept it as part of their diet.
You asked about life span- They can live up to 20 years, although if not cared for properly, the average pet lives more like 10 years- still a long time! They are prone to fatty tumors as they age.
I've left my alone for a weekend, but wouldn't go much longer than that. If I'm leaving for longer I have someone check on them and change their food and water.
Another consideration is that only the males talk, and only some of them.
You said your DS has allergies, has he been around birds? The feathers and dander can be problematic for people with allergies.
About rodents- I think rats are GREAT pets. They have nice temperaments (get a young one) and are very clean. When we had them they were like little dogs- they all come running over to the side of the cage for chin scratches and dog biscuits when we entered the room. Boys tend to love to cuddle. They can even learn tricks, or be litter box trained. They have short life spans and are large enough for a 4 year old to safely handle (but you do need to watch their tails- they are delicate and they should not be handled by them) They aren't smelly and can be left for a weekend. With rats the best feeding option is rat pellets. I really like Oxbow Regal Rat for food. They are omnivores and will greatly enjoy your leftovers as well- they even eat chicken bones (and love them- they are safe for rats). They do require a fairly large cage. This site is great for rat info http://www.ratsrule.com/
They have a cage calculator, and I've also made their homemade rat diet, which could be fun for the kids. Rats should also always be kept in at least pairs- they are high social creatures.
Hamsters are another good choice. They are smaller than rats, but have about the same life span. Some can be skittish, but most are good tempered. I would not recommend any of the dwarf hamster species for young children- stick with normal hamsters. They generally eat seed based hamster mixes. They are not smelly. I should qualify that, I find female hamsters have a distinct musky odor, but many people do not notice it at all. If it worries you, males have no odor. Hamsters can also be litter box trained. A good silent wheel brand is called the Silent Spinner- well worth the money. Avoid wheels with rungs or wire mesh- I've seen numerous hamsters really scrap up and bloody their legs on those. For a noisy wheel, a little veggie oil on the axle really helps. Also, hamsters love to make nests, but avoid cotton wool type nest products- they can get wrapped around legs and stuck in cheek pouches. Plain toilet paper works fine and is greatly appreciated. They, and all rodents, should have a water bottle as bowls will quickly become soiled. Hamster MUST be kept alone. In pairs they will fight, sometimes to the death. (occasionally they will live peaceably, but just as often they appear to do fine, and then turn on each other one day) Hamsters in stores can be kept in the same cage because they are young- after they hit a certain age they become territorial. Hamsters should also have something to chew on in their cage or their teeth can overgrow. You can buy little wooden sticks for this purpose, but many hamsters wont use them, I find small edible dog bones like Greenies or Nylabone Edibles work much better. Actually, I had to ration the greenies because my hamster liked them so much. Also like parakeets, most cages on the market are too small. a 20 gallon long aquarium is about the minimum size. However, it is very easy to make a large and wonderful cage out of a clear plastic storage bin. I can help you with that if you're interested. This forum was always really useful to me- http://hamsterhideout.com/forum/index.php?act=idx