I think the Parents With Prams should be labelled the same way as on trains: Reserved for people with special needs.... and have a sign with a pregnant woman, a wheelchair, a pram and a person on crutches. If a heavily pregnant women has her toddler with her and decides she can't manage taking a pram AND pushing a trolley at the same time then she should be able to use that Parenting spot regardless of whether she uses a pram. I think the terms should be broadened because a person on crutches probably should have precedence over a person with, say, a fit and healthy 4yo who can walk but is popped in a pram.
People using crutches, a walking frame or walking stick are entitled to an ACROD sticker and therefore to disabled parking bays. I was with exP who (despite the fact that I told him not to several times) parked in a disabled parking bay and a woman with a mentally disabled daughter came up and yelled at him because she in fact has an ACROD sticker and wanted to use the bay (and of course exP didn't as he's not disabled). So even having a family member with a non-physical disability entitles one to an ACROD sticker, I gather.

I think it's rude for people who don't have children to use the 'parents with prams' bays, but plenty do. Unfortunately, it's not enforceable and it's hard to tell who has a lil'un and who doesn't, since some people have baby capsules that they take out of the car to carry the bub in, as has already been said. I get very unhappy when I see sports cars with only two seats in parents with prams bays though And yes, I agree that "parents with young children" would be far more appropriate.