I would definitely say no to having more than a couple 3 year olds at a pool party! I'm a little of a freak about the water, so I get nervous. Even the 9 year old, I'd be nervous about. Even when there are many adults around, it doesn't mean they are paying attention. Anyway, I say, if you want to invite some friends of your 3yr. old's that's fine. But, I've heard that the number age your child is, is how many kids should attend the party. So, if your son is 3, 3 kids would be plenty. If your 9 yr. old is ok with combining the party, you can invite a couple friends of 3yr old, make/buy a separate cake for him and call it a party. Even if he's talking about it now, he won't remember his NON birthday party at age 3, when he's 12. Make sure you have plenty of supervision at the party and it could be a fun time! I would also only do this if your 9 year old is ok with it. With kids as young as 3, they are ok with cake and a quick "Happy Birthday" song. It doesn't need to be something spectacular. Make sure you let your 3 yr. old know that next year probably won't be a pool party ( unless this year it's such a success, they all want to do it again). Next March you can do a bday party with family and a few friends around his birthday. I'm only saying this, cuz at some point your older child may not want to combine parties, Good luck and have fun!!