Its not a bad package for an Expat. BUt with a wife and 3 kids its modest. Depending on his Visa status, in my experience, spouses of sponsored expats generally do NOT Get a work permit. Depending on where you live in BKK, rents vary from ridiculously cheap, to the obscene. So check your rent options. Often I see the 'rent allowance' is meager.
Western Schools (ie BIS, AIS) are very expensive in Thailand, this is the most common complaint I hear from expats is that they can not afford to put their kids into International schools. There is no free education for expat kids, even in government schools, and the fees vary a lot. But, unlike US and UK schools, there is little support for foreign students, especially if they dont speak Thai. It could be very challenging for your kids, sure a amazing experience, but initially a nightmare.
With a young child, consider this. If your little one develops a cough and fever, you have two choices, wait it out for 5 hours at the local public hospital, and probably be told to give him/her some paracetamol, or turned away and told to come back tomorrow. Or, spend THB5,000 to 8,000 for a consultation at a private hospital. A lot of expat health insurance policies do not cover general doctors visits and health care, they are all about accidents and hospitalisation. These are the hidden costs that could cripple you.
Really do your research well, do your sums and a budget, reach out to other established Expats and get the real truth. Foreign companies tend to support incoming expat workers well. Not so Thai or other Asian employers. REally do your research.