When I put my grandkids in the tub it was clean. When I got them out there was something black and sticky on the bottom of the tub and the bottom of the kids (not a lot). It wasn't what small children usually leave in the tub. My house is only 2 years old but I've had considerable problems with the toilet that is about 6 feet away from the tub. I had to plunge the toilet while the kids were in the tub. Is there any possible connection?

It sounds like you have a partially plugged drain, and when you plunged the toilet while your kids were in the tub you forced the sludge up the tub drain, you should have the drainlines rootered out from the toilet to the main sewer line. Good luck

Superplumber nailed it.
Your plunger acted like a pump pushing water against the clog until it backed up into the tub drain. You have a blockage downstream from the toilet that must be snaked out and cleared. You have three places you can snake from. 1) you can snake from the lavatory roof vent. 2) you can remove the trip lever assembly and snake through the tub overflow or 3) you can pull the toilet and snake the branch through the open closet bend. Your choice! Good luck, Tom