SHC has been examined many many times, and pretty much always demonstrated as something like a 'human candle' type effect. It's a telling fact that in almost every circumstance there is an open flame/source of ignition of some sort in the room. That has in fact been demonstrated under controlled circumstances with pig carcasses. That is not to say that that explanation is the only one however. But no-one has ever explained how a body can just ignite by itself - not saying it can't, just saying that to my knowledge no-one has demonstrated it can.
Unfortunately, in this instance however, I'm going to plump for the simpler explanation. Munchausen's Syndrome by Proxy. It's much more likely that the parents are abusing their child to get attention than the child is just bursting into flames.