Not sure of your dd age, but if she is a newborn, your ds might be booster ready around age 5.
I am assuming your DS is forward facing?
How mature is your DS? When do you think he will be ready to be in a booster (he will need to hold still, not mess with the belt, not unbuckle, not lean over, etc...)? Kids are generally ready to be in a booster around 5-6, but some kids a bit earlier and some kids much later.
For him, you want a Combination seat--- it's a seat that is both a forward facing harnessed seat and a booster. Some will be the last seat he will ever need, other's need to be replaced with a dedicated booster down the road. One of those would probably work in your situation since you could pass the combination seat down to DD when DS was ready for a dedicated booster.
Because of DS's weight, you'll want a combination seat that goes to a minimum of 65 pounds. He's predicted to hit 50 pounds by 4.5, so one that only harnesses until that age would not work. That said, most kids outgrow carseats by height, not weight. A forward facing seat is generally outgrown when the child hits the weight limit, hits the height limit, has their shoulders go over the top slots of the seat OR have their ears go over the shell of the seat. Because most kids outgrow seats by height instead of weight, making sure to get a seat that has tall harness slots and a high back are very important.
If you think that DS will be ready for a booster by the time he is 5-6 (and he has an average sized torso), one of the more commonly recommended seats should be fine. The Graco Nautilus and Harmony Defender are both well liked seats that function as harnessed seats, then boosters and then backless boosters. Some kids, especially huskier kids or boys, don't like the crotch strap on the Nautilus, so you would definitely want to try him in it before buying. The Harmony Defender has a known short crotch strap BUT they will send you a free longer one if you call and ask.
The Defender is narrower, so fits next to other car seats or in tight situations better. The Nautilus comes in a variety of prints/ fabrics, so some people prefer that. The Defender, unfortunately, is only available at Wal-Mart so if Wal-Mart is a deal breaker, you'd need to eliminate that.
If your son has a longer torso OR you consider him more immature, you would want to consider the Britax Frontier. It has 1.5-2" higher top slots (which gives an extra couple years) as well as higher height limits than the other two. It is also more expensive, though.
DS is a great passenger and not a problem at all in the car. He HAS recently started to unbuckle his chest clip - not to be naughty but he seems to just like to play with buckles and now has figured out how to unbuckle it himself.
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