Take a moment to decide if you want to go in this direction. Plenty of posts on here about people that have split up but the ex kept the phone or racked up big data bills etc. and the account holder was responsible for all charges. Not saying that's your future but it is one possible future.
What sort of data are you looking at a month for them?
I would suggest instead buy a prepaid long term card. That has a set value. You (or they) can also top it up themselves.
I suggest the Boost Mobile Prepaid $300. Give you 240Gb for a year (and unlimited calls). So essentially around 20Gb a month.