Here is a quick gak recipe that your kids can make practically all by themselves!
Recipe from Crafting Creative Kids
8 oz school glue (Elmer's)
1 tsp. borax
3/4 c. water
1/2 c. warm water
Empty glue into a large glass bowl.
Fill empty glue bottle 3/4 of the way with water, screw on lid, and shake to get remaining glue mixed.
Pour into bowl with glue and stir.
Add food coloring until desired color is reached.
In a separate bowl, mix 1/2 c. warm water with borax until slightly dissolved.
Pour water/borax mixture into the bowl and stir (I found it easiest to use my hands after it becomes hard with the spoon).
Fold out onto wax paper and knead (like bread) until smooth and elastic.
You can add more water if it is too hard/lumpy, or sprinkle SMALL amounts of borax into the mix if it is too runny.
Store in a resealable plastic bag.