Well, the whole thing is 3-4 days and is quite fun, know of lots of families who enjoyed this, but whatever suits you. After all, this is your vacation and you should enjoy it and spend your time however you wish.In terms of getting certified in Thailand, that'd be great, but I would only want to do the open water dives in Thailand, and the elearning and pool courses locally so I don't waste time doing this while on vacation
I don't know what exactly you did in Belize, but there shouldn't be a problem teaching a 10 year old kid (and even much much younger ones) to equalize properly.My son is less interested in diving than I am, but perhaps because he had a hard time equalizing when we went diving in Belize. I certainly enjoyed it. He's also not so interested in getting certified right now, but I might be able to convince him.
About convincing children to scuba, we never tried to convince our kids, although it is a family thing and we have been diving every other weekend since before they were born- or one can say they were into this. But we never pushed them and took them diving only when they asked for it, usually around the age of 6. All of them eventually did their certifications at 12, but my daughter is nowadays less interested and prefers other topside activities, so we let her be. Maybe she will come to her senses, may be not. But I think never push or convince, it has to come from them