If partner is the one who drives, then there may be an issue there - he is possibly unwilling to put himself out for you? So she wants to see you, but sorting out children timetables with him to make a move may be difficult.
Plan long term for a visit date, so perhaps just 3 a year, plus one around Christmas - 2 each way. Suggest to her that as things are so difficult to arrange that you fix a schedule. If she cancels one, then tell her, never mind she can come to you on the next fixed date.
Yes, with you working long hours, you have issues, but she is working and has to organise 4 of them with very different timetables. Perhaps she does not understand your lack of time either (perhaps his mother has more time or does not work). As our working lives get longer, it is not so easy as it was when we grans retired at 60.