I'm not sure what your AIBU / WWYD is really.
Accidents happen occasionally. Your toddler didn't meant to throw the ball over, you try to prevent your older children doing it.
You didn't ask for the ball back, they didn't throw it back.
They may not have noticed it in the garden before the dog got it. They might have thought you'd come and ask, or send one of the older children, and they were going to let you go and get it when you did.
They might not have thought the dog would get the ball.
They might not have been sure which garden it came from.
I don't think anybody was being unreasonable really. Although if it was a favourite toy I might have made the point of going to get it rather than wait for them to throw it back.
We used to live in a house with a garden that backed onto someone with three children.
Lots of things used to come over the fence. We used to drop them back over as soon as we saw them. But more and more stuff used to come over, sometimes we'd just drop it back and it was thrown over again on purpose like a game. We started to leave it to the end of the day and drop everything over in one go once they were inside.
By the end of the next day it was all back over with more added.
We spoke to the parents when I was hit by a metal car that cut my shoulder as I put the washing out.
They refused to do anything to stop it.
Then they started to throw stones and bits of concrete over. These were newish built houses so there was a lot of rubble about in some of the gardens around the edges of the fences.
We spoke to them again and they didn't care.
So we stopped returning the things that came over.
Eventually, when we had three pairs of shoes, the TV remote, a handbag, a large pile of concrete, a set of hair straighteners, a radio, and a set of car keys, along with most of their toys and some of their clothes, the man came to ask for it all back.
We told him to speak to the estate manager (armed forces married quarters) as we'd dropped it all off with him earlier that day.
Not one thing ever came over that fence again.
If your children were doing that I think they'd have good grounds to let the dog eat it, but as a one off I think it wouldn't have hurt to ask for it back/throw it back over if they knew where it came from.
But I don't think anyone was especially unreasonable in this case.