I'd start by leaving food there for the momcat - then she will learn to stay closer.

You can also see about borrowing a livetrap, though I would hate to scare her with kittens that young.

I'd say feed her for several weeks, til the kittens are older...then live trap her. By then, you might get to see her and judge how tame she is.

This means you have to get Grumpy to allow more time.

She will likely see you setting out the food also!

Keep posting about this...

Someone will have some great ideas here!

Catty1