Wow. I agree with your DIL, completely. Your grandchildren deserve to have their father around. Not to mention, they deserve a mother who isn't scrambling working full time and raising them practically alone. A law degree is not what it used to be and sounds not worth it at all to me.
She doesn't want you to babysit because the pressure you put on them has put her in a terrible position.
Wow, indeed. And as a lawyer, I totally agree. It does not even seem like your son actually wants to practice, just wants a degree for the sake of a degree and a bump up in his current job. If the latter is the case, I would seriously suggest he reconsider the next 18 months. Their lives (her life in particular) sounds miserable.