E.W.
I had a friend throw a 4 year birthday party for their daughter in June and did a Luau theme. They don't have a pool so instead they bought a few slip and slides, setup a blow up pool, had water guns, a sand box, hula hoops, and a station for blowing bubbles. They cooked hamburgers and hotdogs and everyone hung out outside, it had to be one of the coolest parties ever (I thought). The kids had a great time because there was SO much to do! That kind of party would fit in perfect with both sex and ages.
If you need custom invitations I would be happy to help, www.WorksofaWink.com . I also do coordinating cupcake toppers and favors to match. We could even include pictures of both kiddos, one on one side of the invitation and one on the other =)
Good luck, whatever you decide I am sure it will be a blast!