I make a deep search online. And i find there is no way to fix crosses eyes except taking an surgery. Also, to help them, you can also use eyeglasses to help them, the eyeglasses can make the crosses eyes relax. Also, wearing eyeglasses can make them less noticeable, However, it can't cure cross eyes.
The crosses eyes can also be known as strabismus. They are caused by imbalance eye muscle. And in common, people use eyeglasses to help them. But there is no effective way to cure it. Also, i heard that Botox injection can be helpful to help crossed eyes. Anyway, you'd better consult an eye doctor for the seek of the eye health of your kids.