hi C., i read all the other responses, maybe im wrong but i just dont understand why you cant have a party for both kids, the more the merrier. i never understood why everything always has to be so seperate and big deal. you are a family, it should be celebrated. i just went to a party for a kindergartener and her 5th grade sister at one of those pump it up places, everyone had a blast. and if you really want a theme, let your daughter pick since the baby wont know the difference, or do something neutral like animals or loony tunes or something. my kids are 2 through 6, our friends and family have kids of all ages, it can be easier than you think to please everyone. we did a jungle animal party one year, it was so cute. i got some of those blow up animals to put around, bought a couple of those little pop up tents, let the kids make their own animal masks, i decorated with some jungle stuff, hung lots of green streamers from the ceilings, and everyone had a great time. do some open ended crafts that everyone can do, make cookies or cupcakes with animals, etc... play the old fashioned games, pin the tail on the lion, do a limbo, sing that alligator song with them, play some music, etc... or how about a winter theme? snowflakes and sparkly things, snocones, lots of white streamers hanging from the ceiling with glitter and snowflakes, etc? or how about a winter luau to forget about the chilly outside?? your daughter might get a kick out of that! the baby wont know the difference, he will either have a great time or sleep right through it whether its a shared party or not! have a cake and sing for each of them, everyone will have a great time. dont overthink it, just have fun.