I don't have any experience with this, but I believe a lot of children have used it. I haven't heard of major child-specific problems.
Any silver fillings should be replaced before starting.
Follow the protocol the way that it's laid out, (lower dosage would likely be appropriate for a child) and I think you'll be OK. If you search, you may find child-specific modifications or advice about using the protocol.
One bad experience I read about was a mother who was giving DMSA to her autistic son. She broke open the capsules to put in his drink. DMSA is one of the most disgusting-tasting supplements out there. Of course the child wouldn't drink it. Definitely keep DMSA in its capsules.