"Cote I have never been to a disabled toilet that isn't unisex"
And I have only twice seen unisex disabled toilets - at airports. Where I live, there is one inside the women's area and (I assume) another in the men's.
"The probability of a child being abducted while it's mother urinates in a public toilet within sight or sound of the child is minute."
Agreed. But not impossible.
"The probability of my OH soiling himself because a parent has decided to use the disabled toilet so they can fit their pram in the cubicle is much higher."
The probability of your OH ever finding himself in any toilet I've ever been to is zero. The probability of not just any disabled person but one with severe incontinence issues needing to go to the toilet at that exact instant is negligible.
Even if so when you look at consequences (the other component of risk), your OH soiling himself would be unpleasant, embarrassing, and troublesome. A baby being abducted, as I'm sure we can agree, is far far worse.
Anyway, this isn't about whose worry is more justified. When disabled toilets have changing facilities in them, it means they are not only for the disabled. There isn't much you can say to argue against this, although I'm sure some will try.