You may choose to reinforce your knots with glue. If you keep glue off of the mobile hanger, then you’ll still be able to take the mobile apart later.
If your mobile hanger is a stick or rod, balance it on your finger to find the spot where it balances perfectly, and tie another piece of twine at that point, making a loop to hang the mobile.
If your mobile is on a hoop, tie three pieces of twine each a third of the way around the hoop. I have a partner hold the mobile by holding the strings together in a pinch grip, then I straighten the hoop by pulling on one piece of twine at a time through my partner’s grip. When I’ve found the point of perfect balance, I tie all three pieces of twine together just above my partner’s grip, then make a loop for hanging with the excess.