1

Most places will be proper. There are children everywhere in Thailand and it's a safe country... Chiang Mai, Pai, Chiang Rai, Mae Hong Son, Koh Samui, Koh Tao, Koh Muk, Koh Lipe... etc.

2

We just travelled for five weeks in Thailand with our son who was 21-months at the time. Honestly, we found everywhere pretty easy. I was surprised at the amount of restaurants we found high chairs despite being told in advance that Thai's don't really use them. If your kids like to eat chicken and rice they'll be fine. We found it easy to ask for food that wasn't too spicy. It's easy to find nappies and milk in the supermarket and at 7-11s.

We went to Bangkok, Kanchanaburi, Koh Samui and Koh Phangan (as well as spending a week travelling overland between Samui and Penang in Malaysia, stopping in small towns on the way.) All was pretty easy with a kid. Chaweng beach on Samui was great for little kids - calm, shallow water that was bath tub warm. Perfect! The beach is very developed though. In January you're better off heading to the West coast beaches though. I've been to Lipe and probably wouldn't take small kids there as it's too far from the Mainland and access to medical care. I don't like being more than an hour away from a hospital as kids can get sick so quickly and injured so easily! I'd look for a nice quiet beach somewhere in or around Phuket.

You'll get a lot of attention with twins. The Thai people love children and babies. :)