Good birthday pary game ideas for 4 year old plus friends ranging in age from 2-5?

I was going to set up a swimming pool with water guns and water balloons for them to play with and just let them go wild. I was also going to make sure every kid got a treat bag too. Is that enough or should I have some actual structured games set up too? Mind that I am also entertaining all the parents and other adults too, plus, by that time I will have a newborn baby to take care of amidst everything! This is a little new to me, it's my daughter's first birthday where she has friends invited. She's recently become little miss social erfly and I want to show all the kids a good time!