In reply to jkarran:
> They're fine if the architrave is fine. If it's been put up with three panel pins and a smear of decorator's caulk then they're fine until they're not.
This is fundamentally the issue - they put a loading (not the user's full weight, but part of it) onto something that is not designed* to take any loading in that direction. It might hold the weight but it isn't designed to do so. FWIW I was once in a bit of a habit of doing pullups on doorframes (when younger, smaller and lighter) but stopped when I once bent one such that it nearly came off.
This is to be fair also true of mounting a bar to the vertical parts of the doorframe, but most materials are stronger in compression than bending so there's less likely to be an issue.
It'll probably be OK, but as a failure could cause serious life changing injury if you fall awkwardly, "probably" may not be enough, particularly not for a kid.
* In some cases where there are bricks above it it might be so designed, but not every house has bricks above doorframes (mine for one doesn't, it has a small window instead), and of those that do plenty will have a proper lintel there so the doorframe is just decorative and to aid in the door closing cleanly and with a reasonable seal.
Post edited at 12:15