Hi S.,
I think the best thing for kids so young is just to have several things available for them to do as they want to. For example, a table with crayons, balloons to play with, if you are outside, you can do bubbles, waterguns, just things that are put out that they can entertain themselves. I have 5 kids going on six and I used to do all their b-days separate and starting last year we decided to do one big blow-out every year to invite everyone and do it all at once so we were not constantly trying to figure out different themes, games, etc. I had over 60 people at my house last summer which seems like so much and it actually went very smoothly. We rented a moon bounce (which all ages love) had things all over for them to do, water balloons, water guns, chalk, and the kids were entertained and happy. The parents were able to eat and visit with each other and seemed to also have a good time. The hardest part of the party is when I brought out the pinatas because its not easy to have little kids line up, wait their turn, stay out of the way and so forth. With the little ones I have found that anything too structured seems to just cause stress for me and them and to just let them play and do things at their own speed works the best. I wish you luck with your party, do what you think will work best for you and the people you are inviting, it is supposed to be fun, try to remember that and not stress too much.