If you are having a hard time getting enough phe into your children's diets you could try adding regular milk into their formula. If you use dry powdered milk it is 17 mg per every gram. I do not have a phe book handy right now, but if I remember correctly 1 ounce is 50 mg of phe.
If it is the metabolic PKU formula that you are having trouble getting it try not to stress. There are going to be points at which your child does not get all of their formula in. As long as this is not a problem for a long period of time they should be okay. As always talk to your dietitians or doctors if you're really concerned about it!