Son has a commencal Ramons 16″ bike.
Bottom bracket is crappy, description on commencals site is BSA 68mm, ball retainer.

Can I replace with a square taper shimano 68mm ? Assuming the kids crank will mount on a square taper.(haven’t taken it off yet to check)

1/2″ 1/8″ chain, no reason why I can’t use the spare 10speed in my box? From what I gather the internal width is still the same, just the external width has got narrower on 10/11spd chains?

Take off the cranks and measure the axle width and I’d double check it is actually a 68mm shell rather than believe the online spec.

Shimano now only do un26 ok and very cheap or un55 still cheap but better.

Sadly the un73 pictured is no longer

She’ll is 68mm
Estimate on the axle length 118mm

If you don’t have the tools to fit the new one or remove the old one it’s worth getting a quote from your lbs.
at work we fit them for the rrp cost of the bb around £25.
That’s £25 total inc the BB