I think if T is consistent most of the time, she should be allowed to cancel for a personal day if she told you with a few days warning. She is, after all, a real person too and needs "mental health" days. I'm a teacher and even though I feel guilty when I take a personal day and leave my kids with a sub - sometimes I just need a break or something fun does come up and I want to use a personal day. I'd cut her some slack if she's consistent all the other times.