I think a clear arrival time is completely reasonable.
It does not need to be 45 minutes early. For U8, something like 15–20 minutes before kick-off is normally enough to find the pitch, get boots on, say hello, get moving and hear any simple message from the coach.
The key is making the expectation predictable.
If one family repeatedly arrives at kick-off, speak to them privately rather than making the whole group stand around for a lecture.
I would also avoid punishing the child if the late arrival is outside their control.
From the coach’s side, do not make the pre-match so complicated that one late player ruins the plan. The football should still work if somebody walks in with ten minutes to go.
A reasonable arrival expectation helps organisation, but grassroots football should still feel like football, not an airport check-in.
