Caveat,
Family law varies from state to state, but when it comes to children, the laws are fairly close. You may want to conduct a Google search on Parental Rights, Family Law, etc. for your state.
Example (Tatoo law): I just conducted a search in Illinois on tatooing children under 18, which requires parental consent in Illinois. (http://tattoojoy.com/tattoo_laws/united_states,illinois,1.htm) I could not find anything immediately on religious baptism. But, I cannot imagine that a parent could stop a minor from being tatooed, but not from joining a religion until age 18.
FINDLAW.COM is a good resource. I looked up Family Law and here is a list of areas where the law legislates in Illinois. You may want to type in your state and see if the topic of religion or baptism comes up. Usually, the law leaves the issue of religion up to the parents to control, unless there is a disput in custody, and thus which parent's religious affiliation will prevail. - http://family.findlaw.com/
If you have a serious legal issus, call a Family Law attorney in your area.
Jim Whitney