The IRS is not going to give you much more than you have already received. To try to figure out which child, the rejection might say something like "Qualifying Child 2" at the beginning of the error message. If it does, who is the 2nd child listed on your return? That is the child that is at issue. If it doesn't say that, are any of your children old enough to work, did they get a W-2 for their work and possibly file their own taxes and claimed themselves.
Double-check and make sure that you entered all of the social security numbers correctly and the birth dates are all entered correctly.
Either way, unless you made a typo in a social security number you will have to paper file. A typo in the social security number of one of your children is the only way you will be able to e-file.
