I would use just one of them now, and save the other for something in the future.
Ok, first off, I would get some some featherweight fusible stabilizer (like this stuff:
Pellon) Gently fuse this on the back of the piece, it will help stabilize the fabric for you, especially if it is very thin. It is what I use to stabilize silk.
Then, hmm,....well you can always add borders in graduated sizes......but as a mom
I might like to have some blocks too....especially made by my son.
So if one of the tops is 35x42....a general size for throw/lap quilt is 50x60 (ish, mine just end up being what ever they are when done ) then you need to add about 20ins to both measurements, if you split that btween the two sides, then you have 10ins per side to fill. So if you do 8in blocks, then add a small 3in border, it will lap size. Perhaps pick up a few fabrics that you like and a few that your mom would like, and do some type of 4 patch or even a pinwheel. That way the top is a combination of everyone's likes
and as a mom, that will mean a whole lot!
I would also keep the quilting very simple, stitch in the ditch or even do a stitch in the ditch about 1/4in from the seam on both sides......that will keep the focus on the fabric....