Payload and Axle loading is usually more of a limiting factor than published tow capacity. Add the weight of all the people and gear plus the hitch and tongue weight and make sure you are under the payload/axle weight ratings.
A quick google search returned multiple payload ratings for the Envoy. Either the web has it wrong (
) or there are different ratings for different models.
Remember - your little ones grow up fast - I'm assuming you will want to use the vehicle for a number of years, so use their weight in a few years time for the calculation.