Just south of Phi Phi is a larger, cleaner and much more family friendly island called Koh Lanta. We stayed at Lanta Family Resort, right on the beach, and there were a lot of people with kids there, and the price was definitely right! It's very calm, quiet, none of the raging discos and spewing backpackers that are rampant on Pha ngan and PP.
Koh Samui, similarly, is a lovely place, a little bit more wild and noisy than Lanta, but easily escaped, and the beaches are great, and there's a lot of people around, which means you've got all the services you could want.
Why don't you just check out Air Asia's fares and go for whatever is cheapest/most convenient?