back when my daughter was little and there was always too'ing and fro'ing, shuttling hither and yon, it was IMPERATIVE to me that anyone else who was slated to pick her up and deposit her somewhere was absolutely RELIABLE. which is why my own mother was not on my list!!! since you cannot Trust his state and since HE is not able to maintain a sober state when it's his turn to pick up the children.....
TAKE HIM OFF THE LIST.
if you are truly thinking into the future, and issues like custody, and he's ON probation AND drinking....and you have concerns about him driving with the kids......tell his PO. not to be a bitch, but to protect your kids now and in the future.
slightly OT - but i can't imagine taking 3 xanax a day and doing anything but drool on myself!!! AND drinking? sheesh.