To do child’s pose, I typically tell my students (from downward facing dog) to drop the knees on the mat and apart (so you have a little cubby for your belly), toes touching, and push back … sitting on the heels.
To take pressure off the knees you might need to use props. Put something (a thick blanket) and sit on that instead of directly on your heels.
If your knees are “tight” and you feel a pulling in the joint, the blanket should help. But if your knees hurt because they are pushing into the ground, that’s another thing - maybe you just need padding b/n your knees and whatever you’re practicing yoga on!
I hope that helps!