Is there a chance that ‘pooling’ the accounts has caused something odd to happen with the text counting?
A few years back, my sister (probably the same age as the OPs daughter actually!) had a contract with, say 2000 texts a month. She used to use almost all of them within the first two weeks and then ration them to last through the last 2-3 weeks of the month.
One month she was eligible for an upgrade so went into the T-mobile store and switched over to a new phone. This changed the start date of her contract month.
So, previously, her contract had started on the 1st of the month, she’d used 1500 texts in one week, then had gone in and restarted the contract with a new phone, and a renewal date of the 8th. So her 2000/month text allowance was pro-rata’d down, to roughly 500 texts for a week. Which meant that she was retrospectively charged for the 1000 texts above her ‘allowance’ that had been used. Cue a bill for >£130.
It took a while before we realised how the bill had got so big. She was good at checking her balance/allowance, but when she’d used the texts they were free. it was only when she went into the store and got a new phone that she was back-charged for stuff that was free when she used it.