You might want to do it outside on a nice spring day. You can put decorations inside these eggs or leave them empty.
Parental supervision is recommended.
Put colored crochet string in a gallon size bag and put stiffener in it with a bit of water to dilute just slightly. Mush mush mush. Pull string out a little bit at a time out of the bag, using your fingers to squeeze excess stiffener off. Let dry on something like wax paper or plastic. Or you can just hang it or put it on the table as is.