Yep.. What he said. They just shred stuff while thei chewing teeth come in.
I used this feeding chart and schedule http://www.orphanedwildlifecare.com/squirrelcare.htm#11. I liked having something I could print off and look at offline and a basic guide. But I fed to the 5-7% body weight . The suggested feed size in this chart seemed high to me. Or maybe I just had small squirrels. I went on the premise that it's better to under fed than over feed. (Bloat or diarrhea )
Once I bought " wild mammal babies " I found its chart to be similar . 6 weekers being fed 4 times a day ( formula) and 7 weekers 3 times a day.
I did not "force " wean but I did eventually take them down to one feeding ( 10/12 weeks) as they were still not terribly interested in solid foods.
They did however enjoy crunching wood and bark and antler.
I assume your keeping them till spring ? I have no idea about overwintering .. But I would not force wean them . I assume that they will give it up by 20 weeks ... But I would try to keep them on formula if your overwintering .