T.H.
Really you're only doing the 7 month old's basked for the sake of the 4 year old, so put in there stuff that the baby needs (and that maybe you have already). I'm putting in my son's (6 months) some diaper cream, soap, a book ($1 at Target) and a baby stacking toy (also a $1 Target find). I might even put in there some baby food like beans or peas since he can't eat they candy yet!
For the 4 year old put in some fun things that might get him/her excited for spring like sidewalk chalk and bubbles. Also books, coloring books, crayons, markers and stickers are always a safe bet or maybe some sort of card game or small toy. Also you can fit in some basics like bubblebath or socks and fancy underwear with characters on it. Do just a little candy and call it good.