Oh how cute and special!
As
@thimblebug6000 mentioned, each batting has requirements for distances for the quilting. So start there. However, I suspect that this will be such a special quilt, that it will not be heavily used ... and may instead be more of an art quilt, to hang on a wall. If so, the distances may not be quilt so important. Although, even a hanging quilt can start to sag, if the quilting distance is too far apart.
This is similar to what I have to figure out when doing some of my embroidered quilts. Sometimes I outline stitch around specific items/features of each block. In your case, that might be to stitch around the Christmas tree, snowman, gingerbread etc. Not knowing the size of each of your blocks, that may be enough. Or choose a second item. If there is nothing else, you could still add something by making a star or a snowman with your stitches etc. When I am at a total loss, sometimes, I do a comic bubble or cloud sort of a bubble around the picture part.
When I am at a loss, and this is hard to explain ... sometimes I draw lines from corner to corner and stitch on them until I get to the picture and stop. Then do the same from the centre point of each of the sides to the other. Also, will do at intervals along the edges. What you get is a nice sort of exploding start outwards, or bringing your eyes inward to the featured work. Wish I knew how to explain it better! Maybe someone else can? And this is where I wish I could share a photo ... I gave up on that process a long time ago.
What I am interested in learning from you
@Jaclyn080208 and I am sure many others are interested too ... is more about the process of how you had the kids do their blocks. I think it looks as though you had them paint directly on fabric. What fabric did you use? Paint types? Any special process to stabilize/protect? etc. etc. And I may be all off base here with my assumptions! Please? and Thank You!!
Merry Christmas!!!