What is the best visa option for visiting family in Thailand for three months?

The advice people gave you about getting a single entry tourist visa from a thai consulate in your country BEFORE you wing your way here is sound advice (y)

That visa will get you stamped in for 60 days and you can get a 30 day extension on that at the immigration office for 1900baht once you're here.

Unfortunately there is no "visit grandchildren" type of visa you can get and seeing as your son is NOT thai you can't get the "visit thai family" visa either.

I'd say your best bet (especially seeing as you're only here 3 months) is to get a single entry tourist visa, come here, get stamped in for 60 days and then go get the 30 day extension at the immigration office.

Good Luck, :)

60 day Tourist visa. There is no such thing as a 'Relative Visa'.