Trapping my barn cats

I have a momma cat, a daddy cat, and five kittens (all barn cats). I can catch the momma and daddy, but am having a heck of a time getting the little ones. They come to eat in a large cage, but unfortunately they can crawl out of the openings.

Any suggestions for trapping the babies in a humane way. They are about 4 months old.

The mom and dad are off to the vet this week to get fixed.

Thanks,
Melissa