Have a look at Cha am / Hua Hin .
We only came back from 3 weeks ago , travelled with grandchildren 5 & 2 they enjoyed it .
Stayed at the Holiday Inn Regent Resort , Cha Am , right on beach and in the evening a short stroll from hotel quite a few eateries and shops .
A few day trips can be arranged , see local taxis not hotel bookings , to see elephants and other animals in local safari park , and on way back drop off to see monkeys .
Local night markets in Hua Hin are a must .
My grandaughters were quite tearfull in leaving and keep asking when they are going back !!!!
P.S there was some concern about buying essentials for the children , no problem 7/11 had nappies etc. and in Hua Hin there is a Tesco store .
The rates for the hotel April-October are brilliant , pay for up to 7 nights get same number of nights free , so 2 interconnecting rooms work out well .
Another advantage is that it is in easy reach of Bangkok , do not have to fly which Iam sure you are aware can be stressfull with small children .