Hi Lisa,
If you can schedule a reshoot, then go for it. But at the next session, here's some things to do differently (in my opinion)
1.) Timing - personally, for these ages, I like to schedule the shoot for 9 am. At that point in the morning they are fed and fresh. Expect you'll get only about 45 to an hour of total usable time with them, as they will be wiped out and unruly or tired past that.
2.) Remember, they are only little bittty kids, who don't have the perspective or stamina that you do on the situation. This is very stressful for them, as they have been fussed over by their mom's before you got there, they are being photographed by a stranger, and all this is alot for them to handle, so be empathic to their situation. but don't give up. Use the fact that you are a stranger to them as a strength. For me , I think you have two cards to play :
often kids enjoy the attention, so take advantage of that. I find in looking back at my photo shoots for reference photos, Many of thee ones I selected in the first half of the session, so the novelty does wear off on them, and
As an adult,the children are more likely to repect your right to authority for the first half of the session, and again, their acceptance of that lags the last half. ( I must say however, that in those cases where children are still enthusiastic for the session extends into the last half hour, the expressions become more "them".)
So in order to take advantage of their attention as quickly as possible, I have "composed" the layout of the painting in my head, but I when I actually shoot the photos I will photograph each child separately, knowing I will "photoshop" them togeher later.
It becomes easier to make it a successful experience for you, the children, and the parents.
Here are three examples of paintings of mine by which I conceived through this method:
http://www.heirloomportraits.net/woa_gracia-madison.htm
http://www.heirloomportraits.net/woa_ian-elise.htm
http://www.heirloomportraits.net/woa_alison-charlie.htm
And I also find it easier if the parent isn't in the room. In fact, photgraphing them one at a time is a great reason to keep the mother occupied yet involved.