All little ones regress differently and react differently to various things, so I know it's pretty tricky when they need help to regress.

Pay attention to their headspace

This is especially easier if they have a wide range of little ages (I myself am 1-7). Pay attention to whatever you do, see if it makes them regress more or if it ages them up a bit. Normally whatever makes them regress more while they're in their headspace is what'll make them regress when they're big.

Ask them

To be honest I'm still very unsure how I regress, and that might be the case for other people aswell. Tho it is very important to ask what makes them feel small because they might know, it'll only do good to ask.

What if neither of these work?

Well than it's probably time to test things out. Try giving them petnames (which I made a chapter on) and baby talking them. Change them into a little outfit and put on their favorite little show. Give them their stuffies and some toys, have them color a picture for you. Ask them what color certain things are or how many of them there is. No matter what you do, I'm sure they'll appreciate that you're trying.