A stable car nap is often more valuable than a minor stop
When a child finally falls asleep well in the car, that nap often protects the rest of the day more than parents realise. Waking them too quickly can turn one small stop into a messy afternoon and evening.
So the first question is not whether the family has already arrived, but whether the stop truly deserves the interruption.
Wake only when the stop is core, limited and efficient
If the stop is the day's true headline sight, cannot be replaced easily and is simple to handle once awake, waking can be justified.
This works best when the child can wake, reset and enter the visit without extra queues, heat or long walking immediately afterwards.
Flexible viewpoints rarely justify breaking the nap
Roadside viewpoints and optional photo stops usually do not justify waking a child who has only just fallen asleep. Their value is usually too small compared with the damage to the next few hours.
On a family trip, replaceable scenery is common. A well-timed nap window is not.
If you do wake the child, keep the transition short and calm
Do not turn waking into an immediate high-pressure sequence. Water, a short cuddle or a quiet reset often works better before moving into the stop.
The real cost of waking usually comes from what is piled on immediately afterwards.