P.G.
Gear the party toward the 4 year old cause he'll actually know and enjoy what's going on. Your daughter is too young to care and will be for at least 2 more years.
And if the cake is for lots of non-family people, I'd have a birthday cupcake or small cake just for her cause really, family doesn't care about their own kiddo's baby drool and hands in a cake but it might gross out some non family (LOL :)