You don't want to lighten the tongue to stay within your GVWR, it can lead to instability.
Something you can take into account is the fact that the WD hitch will transfer some of the tongue weight back onto the trailers axles. The amount can vary depending on set up but you could see as much as 20% transferred. At 20% of 900lbs you could see 180lbs removed from the truck. BUT, if you need to rely on this to stay within GVWR it goes to show that the TT is too much. Keep in mind your kids aren't going to get any smaller.