So many things to gather with granddaughters! Give it a whirl with your serger, whether you have a gathering foot or not.
Without a gathering foot serger settings:
left and right needles at the highest number
differential feed (if you have it) about 2
stitch length at 4 (longest stitch length)
Set your fabric under the standard serger foot and go! Serge off, leaving at least 7 or 8 inches of thread tail. Don't cut threads close to fabric or you will lose all that work! Also don't pull on the edge of the fabric, or again - gathering is lost!
On the other hand, if you find you have too much fullness then give that edge a slight pull. Adjust gathers.
If you have a gathering foot, follow the settings given for your machine. You also have the advantage of being able to gather and sew to another piece of fabric at the same time.
Have fun and just enjoy the serger!