Hi debi,
You will love Thailand I'm sure. I've been to Goa and loved it and am shortly going back to Thailand for the third time.
The weather is better in Thailand the earlier in the year you go so if you can go in March, I would recommend that.
I would advise you DON'T go to Pattaya with three relatively young children. I went there last year and the main touristy area is called Walking Street. This is full of restaurants and bars but the majority of bars are Go Go bars, with scantily clad girls offering all sorts. To be honest, it's pretty seedy and there are way better places in Thailand to go for families.
Koh Samui is lovely, I'm off to Hua Hin in a couple of weeks, which is the holiday destination of the Thai Royal family. Evidently they don't have Go Go bars there, just 'normal' bars and restaurants etc, parts of Phuket and Krabi are lovely as are some of the smaller islands.
Price wise, it's a little dearer than Goa overall, but still really cheap. We found this little restaurant last year in Pattaya that did fantastic food and it worked out at £5.40 total for two starters, two mains and a beer each!
A beer is around 60-70p if you stay away from the really big bars, or can be around a £1 for a bottle of Chang (nice beer mmmmm

)
If you eat in one of the better hotels, it can cost £20-30 for two at lunchtime ( we paid this in the Sheraton - top top food, but expensive by local rates).
Cheers
TC