Kids are too young for serious trekking. The month is important too cause that park gets more rain than other places. If you want to keep costs down Id skip Samui and Khao Sok. The tours in Samui are double the price of tours in Krabi and hotel prices are higher as well.
Khao Sok has 2 sections being the lake area and the jungle area. They are a long way apart.
Chiang Mai and Krabi would be good choices for a family.

