I'm thinking about doing a bit of fiddling with found/cheap bits and pieces and was wondering if the hubs / wheels on kid's bikes are strong enough to be used as fixed /single speed for adults? And whether axles widths and OLDs tend to be compatible? Same goes for BMX hubs/wheels.

As far as I can tell kids bikes use wheels and other components that are perfectly useable. I bought a kids mountainbike a few weeks ago for £2, so far I have only used the brake blocks and inner tubes. The wheels would be useable but are unfortunately pink. I might reuse the front suspension forks at some point if I find the right pproject.