Work backwards from the evening
Look at the available time, including choosing the film, opening credits, snack preparation, and getting ready for bed afterwards. If the movie barely fits before adding a break, choose a shorter option or plan two sessions. Tell everyone the arrangement before pressing play. A promised intermission should not disappear because the plot becomes exciting or an adult wants to finish. The plan is meant to make the family activity workable, not to measure endurance.
Choose a practical pause
An approximate halfway point can work, but wait for a scene change if one is close and the child is comfortable continuing. Use a bookmark or write down the timestamp so returning does not require searching through spoilers. During the break, offer the basic things people actually need: a bathroom visit, water, movement, or a quieter room. Avoid starting another absorbing screen activity that makes the return a second negotiation. Ask who still wants to watch the remainder.
Leave room to end for the night
If the break reveals that everyone is tired, stop there. A brief recap tomorrow can make restarting easy: ask what the characters were trying to do and then resume from the saved point. Do not call the unfinished film a failed movie night. The shared activity may already have delivered what you wanted from the evening. For the next selection, note whether the length was the issue or whether a particular section simply felt slow for this audience.