Only thing to do is cut them down low, then apply a full-strength herbicide w/a cloth or brush on the exposed surface of the cut. Try to do it when there will be little/no chance of rain for quite a few days. Even then, some roots will prb'ly survive and then you'll have to repeat the process later. Wear throw away gloves or if it gets on your hands, wash thoroughly afterwards.

It's been discussed several times already: https://www.houzz.com/discussions/1649416/how-to-kill-ailanthus-aka-tree-of-heaven

I don't have access to the trees. Just babies. There's a wall in the way. Is there a way to kill the mother tree via the roots? Like maybe slice open the roots and put roundup in the cut?

Doing what I suggested to the "babies" might be the only thing you could do (it would be about the same as doing what you suggest, but that won't kill the mother tree), other than find a way to get rid of the mother tree -- ask the owner, complain to a local government, etc.

I killed one using triclopyr concentrate on freshly cut stumps.

This was a tree in my catty-corner neighbor's yard- the home was vacant for a couple of years so I took the opportunity, lol.

I had to scale a fence to do so (as well as drag away the felled tree) and I'm an old lady- surely you climb a wall?

HA! cearbhaill. I have done some guerilla gardening too. A few years ago there was a foreclosed on house down the street. I noticed a huge patch of (invasive, Eurasian) thistles growing in a side yard the mortgage company was refusing to cut. They probably blew in from the adjacent farm. There was no other landscaping that would get injured so I sprayed the dreadfully powerful confront onto them one night. Local thistle crisis averted. They all died and were replaced by grass, because clopyralid has a residual anti-asteraceae effect. (anti a lot of other stuff anti-effect, too...you would need to be very, very careful where you used the stuff and it's caused problems with compost in the past. And is no longer labeled for use on residential lawns. Well, this wasn't a lawn anymore. It was a 4 foot tall brush pile. Nobody was going to make compost from that yard any time soon, that's for sure. The new owner's plantings are doing just fine...by the time the house sold, it had broken down.)

And btw I can guarantee you the farmer was nervously eyeing that patch and thinks I did him a favor.

I had to scale a fence to do so (as well as drag away the felled tree) and I'm an old lady- surely you climb a wall?

LOVE THIS!