If a 31 year old mother of 2 gets pregnant by a 17 year old, what are the consequences for the mother and children?

It depends. For starters there likely won't be much child support from the minor father. The father of the older children may end up going back to court to get primary care of the older children based on the mother's inappropriate conduct/mental health etc. At a minimum, if that father goes to court, a parenting plan investigation would likely ensue and a mental health evaluation of the mother may be ordered. Or nothing may happen since you provide no information about the father of the older children and the mother may just be stuck with a very immature father for the new baby.