I lived on Samui for almost a year and there were loads of kids and families about. You could book a guesthouse type place that charges monthly rates rather than a villa/hotel and it would be a good bit cheaper - I stayed at Moon Hut for two months and then the rest of the time at another monthly rental place, both in Maenam and both sizeable enough for a family. Moon Hut is about 30 seconds from the beach. Don't stay in Chaweng - very clubby and full of prats on a jolly!
I'd definitely take the kids there for a month. Do a few days in Bangkok and then get the train and ferry to Samui - it's small enough that you can see it all in a month as well as get plenty of beach days in, go scuba diving, go to Koh Tao etc. And there are loads of Western food places and shops there - Tesco, Boots, McDonalds, Subway - and lots of good pubs serving British/Western food: the Nelson in Bo Phut is fab and excellent value, and you must go to the Lazy Coconut in Maenam for Sunday roast dinner (a bit pricey but a Samui institution).
If your boyf is going too, hire a couple of scooters (do this through your guesthouse rather than just turning up at a scooter hire place as they might try and rip you off by claiming on return that you've 'damaged' it), buy helmets at Tesco, put a DC on each and off you go. It's difficult to get around otherwise, so a scooter is well worth hiring. People go on about the Samui roads being dangerous, but as long as you're careful I found them perfectly fine - and I had never driven at all before going to Samui.
What else? You won't need a rabies injection as it's virtually non-existent on the island and there are hospitals within a quick drive (my doctor told me when I went to get my shots before going that rabies injections are really only to delay any infection before getting to hospital, ie if someone is working in the deep jungle hours away from civilisation, and not to pay the £90 it would have cost).
Clothes - I don't know your tastes, but one mistake I made was taking a tiny backpack and thinking I'd buy clothes out there. Fine if you want to wear cheap floral empire line dresses that make you look six months' pregnant, not so fine for short busty types who couldn't find anything to fit 
. Shorts and T-shirts and flip flops are fine everywhere; I wore short dresses and sandals. If you do need to buy more clothes, or stuff like towels/household goods, the big Tesco just outside Chaweng is best for Western sizes and there are a few shops in the Big C supermarket.
Go! Enjoy!