[wdfifteen]

Trustees typically get paid for their service as trustees. The amount might be stipulated in the trust. Mom can demand to be paid a reasonable amount for managing the trust. As the trustee she can pay herself. It's done all the time so a little research should reveal what a reasonable/legal amount would be for the size of the trust.
How high functioning is the child and how long can her money be expected to last?
States vary, but one thing to consider is that if the child runs out of money and needs government assistance like Medicade, the state recoups its expenses from the recipient's estate once the recipient dies. Making the house one of the trust's assets could potentially mean the state gets the house and mom gets thrown in the street.
I would look at spending down the trust assets to pay normal living expenses. It seems reasonable that the child's trust can pay to the mother the cost of maintaining the child's standard of living. That would include housing, transportation, food, etc.

As stated before, you need an attorney if you don't already have one. I understand coming here noodling for ideas (I do it all the time), but PPOT is no substitute for a professional.