Aug break with elderly and young

[3pteepee]

We're planning to travel with my parents and 5year old DS in August somewhere in the UK. I am relatively new here and haven't explored much. Considering the time of the year, mum's disability (she can't walk much), I am looking to get some feedback on which of these would be good to visit? Lake District, Northumberland, Scotland, Wales, Cornwall, anything else? Sorry, the question is entirely open-ended..

Cornwall and the Lake District will be rammed in August. Northumberland would be my choice as there are some lovely towns and beaches and lots of attractions for all generations.

You've listed a fair chunk of the UK there and some big areas which offer a range of places. e.g. Scotland and Wales you could do city breaks, or seaside breaks, or a week of practically wilderness hiking (which I'm guessing you don't want).
Everywhere will be busy in August and many places will already be booked up.
What sort of break are you looking for?