Can a grandparent be arrested for interfering with custody of their grandchild?

Absolutely! In Florida, Section 787.03 provides that whoever, without lawful authority, knowingly or recklessly takes or keeps a minor child from their lawful guardian, even if in foster care, commits a felony of the third degree, punishable as provided in s. 775.082, s. 775.083, or s. 775.084. This can also apply to parents.

Grandparents who may have  rocky relationship with their own child or the other parent of the grandchild, should have something in writing which allows you to have possession of the child, even if it is a handwritten note signed by the parent. Take that extra step to protect yourself.