The problem with drawing parity in law between being disabled and having a child with you isn’t that it necessarily has a physical effect on parking spaces per say, as you said, it doesn’t necessarily imply that bb spaces would be removed or replaced by p/c spaces-
although actually, if shops were told they had to provide spaces both for p/c and disabled people under the same legislation it’s extremely likely that they would start to label spaces as ‘accessible’ and allow people with kids and disabled people to use the same spaces, because it’s easier and cheaper for them-
this is exactly what has happened with accessible toilets having baby changing in them,
and even with the large scale removal of luggage spaces on busses meaning disabled people are fighting for access (because the buses want bums on seats, and as many seats as possible- so if they are forced to leave empty space for wheelchairs they have taken it back in luggage space, but people still have luggage and prams)…
basically no profit driven entity is going to give up anything if they don’t have to, so the reality is that when one group gains, another loses- wheelchair bus spaces are gained by disabled people but parents with prams loose out-
The larger damage is in the change in mindset and beliefs of the population- disabled people are already bottom of the pile, with many people resenting any support we get: if the government starts legislating for parents with children in the same way they do for disabled people, then the resentment of and feelings of entitlement towards our provisions will be strengthened.
The message will be that nice to have convenience things (p/c spaces, space not to fold a pram etc) are as important as necessary to normal living things (bb spaces, accessible toilets), and people will be even bigger twats about it than they are already.