Hi Kitty,
If you are flying THai all the way to Phuket, they will check your luggage all the way through. Phuket is an international airport, so you will quite likely clear customs and immigration there also and just stay in the transit area in Bangkok. I have never had my hand luggage weighed and as our kids also have a 20kg limit, we never go over. As you are travelling in a group, book your bags in together and even if you were collectively a little over, they are unlikely to charge you. I have been charged excess on Qantas once, but between 2 of us we were about 25 kg over and they charged us for 10. You do need to be careful on the budget carriers though as our last flights were with Jetstar and I did see others being charged.
Also, to answer your question from another Thread, yes, we were fortunate enough to go to THailand twice last year. We went on a business conference/junket to Phuket in August, staying at Le Meridian and we had our family trip in November, 19 nights Bangkok/Phi-Phi/Ao-Nang. We are saving up for a trip to Disneyland next year but I am getting itchy feet at the momment so an October trip may be on the cards.

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I love Thailand this much