With or without the kid, I would not try to stop them, but that does not mean I would have tried to get a grip on my gun and potentially drawn it (with no child present) and kept it concealed just in case they decide they are not leaving after all. That sort of thing MIGHT work if you have been ordered down on your belly and you can move your unholstered gun to underneath you without drawing attention.
While I would be glad to let the folks go, just because they have the money and have stated their intentions are to leave without trouble, that does not mean they are going to leave without trouble. In some 12% or so of robbery cases, according to FBI information from the Discovery Channel, bad guys go ahead and shoot, stab, club folks AFTER they get the goods and everyone in cooperating. Sometimes it seems planned. Sometimes it seems to be a last second decision to just shoot somebody on the way out or in some cases, one of which was shown, the bad guys get all the way outside, and then come back in, cap a couple of folks for good measure and then leave again.
The point is that just because all the requirements have been met and the bad guys are about to leave, that is no time to relax your guard. Contrary to what a lot of anti gun folks think or the grieving parents of a bad guy that got killed, just because they state that if you give up the money and don't cause trouble that they will leave with the money and everyone alive. The argument is presented like some sort of agreement between parties in a business decision. It isn't as if they have entered into some legally binding contract, however, or that just because they have given you their word on their intentions that you would have any reason to believe their word on their stated intentions is going to be truthful. There is no reason to believe people who have robbed you and threatened your life. If they are willing to do that, then no doubt they are willing to lie to you. Maybe it is true and maybe it is just a ploy to pacify you and the others so that they will have less trouble in dealing with you when the time comes.
Interesting about this situation being inside a bank. Why would I want to stop the bad guys from getting away with money that is NOT mine (or not much anyway that is in my account) and is FDIC insured anyway. Why would I risk my life to help the FDIC?