Yeah it's a bit double. Don't think of sad mama cats looking for their lost litter for weeks, think of preventing more strays.
I found the golden middle road with the kittens I found in my backyard. The mom was a half stray, and after a week of telling her not to try and grab her kittens and move them back outdoors*, she accepted the situation and snuck into my house every so often to come and feed them, and even taught them to mouse-hunt** right under my nose. She eventually got comfy enough to allow being petted, and for the two years to come, she kept returning in between periods of heading off into stray land. The lucky 3 kitten buggers that got a pass to stay (I got the other 4 new owners when they were 8-9 weeks) got like 4 months of breast feeding from the poor thing. They were eating 'normal' kitten food as well, but they were just spoiled little brats like that, and mama cat just tolerated it well beyond normal nursing age before finally telling them to bugger off.
Sadly, I had to move from that place (it were old houses and their demolition day had come), and the weeks leading up to the move, she was nowhere to be seen, or I would have put her in a carrier and taken her with me. She was a gentle and friendly stray. I had to console myself with the thought that the neighborhood's crazy cat lady who fed like 45 strays lived in a house nearby that was not going to be demolished.
*) let's just say I got as little sleep as fresh parents with a newborn baby that week, in between opening and closing doors and windows, and blocking the path of a frustrated half-stray with a kitten in her mouth.
**) this is one of the most awesome things I ever saw cats do. Mama cat was sitting in the middle of a (near perfect round) circle of 7 kittens, and she presented the live mouse she had brought to each one of them in successive order, encouraging them to growl and play with it, before finally killing and eating it in front of them. I still regret not having had a camera at hand to film it.
She taught them well. All three of them cats grew up to be good mouse hunters. They just brought one in two days ago. And ate it, didn't find a trace of it, as usual.