For the child -
Coloring book and crayons
A new story book
Matchbox cars (or other small portable toy that he would like)
Healthy snacks like juice and dried fruit (if he can have them - not sure if he's having something done that would prevent him eating certain things?)
For the mom -
Some kind of activity for after her son goes to sleep, depending on her interest - word find book, music, Kindle credit for a book to read, Sudoko, etc.
A nice moisturizer and/or lip balm - hospitals are always so dry.
A nice treat (assuming she can eat whatever she wants) like a really nice chocolate, or whatever suits her taste, that she can have if she has a moment alone.
Apartment Farm - the chronicles of my cooking, gardening, crafting and other such things.
Assuming the mom is staying with the kid, and not a patient herself - Food so that she doesn't have to leave to go get meals (in other words, not just a few snack items), especially if there isn't someone else to stay with the kid. Alternatively, a gift card to somewhere near the hospital that delivers.
Coffee or tea (or gift card for it, but again, you run into the leaving-the-room problem) if she drinks it, as she is likely to get little or no sleep. Maybe a mug and good quality instant coffee (or tea, or cocoa).
Something light and brainless to read, or some other entertainment. Something that isn't going to be upsetting or require too much concentration.
A pillow (in a colored or patterned case) and/or blanket (either for mom or kid).
What you get for the kid is going to depend a lot on what is going on. A lethargic kid, or one in a lot of pain, is going to have different needs than one who can barely be kept in bed. But overall, the previous suggestions are pretty much what I'd say. Crayons and coloring/activity book, simple/non-messy crafts, simple games, legos or other building type toy, picture books, read-aloud books, or audiobooks, a stuffed animal, videos in whatever form they can utilize. A lot depends on the hospital. If it's a children's hospital or one with a large pediatric floor, they probably have plenty there to keep a kid occupied, and I'd focus on a few comfort-object type things. In one that is less well-equipped, they're going to need more in the way of entertainment.
I agree about healthy snacks/comfort food for both mother and kid (if allowed) - hospital choices tend to be standard junk food and vending machines.
DS born 6/03, DD1 born 9/06, DD2 born 10/10, DD3 born 4/14.
Ocelotmom, you bring up a good point about children's hospitals having great built-in entertainment for the kids, so maybe I'll just do a cuddly buddy for him and then focus on comfort items for the mama.