Different calendar dates are calculable and different languages are translatable.
Missing stamps may be a problem. If they don't know when they exited the UK, then wouldn't calculating absences be a problem?
At the time of the missing stamps, do you have any evidence to corroborate as to when they travelled or a maximum time that they were absent? Perhaps you can get these information from the airlines or carriers?
The children's school records would show when they are present in the UK and could not have been
absent (
summary) during school term-times.
If the children traveled with a parent, then could the parent show when they cannot be absent from the UK, due to parent's working or otherwise?
There is a margin of error when relying on
memory. However, after allowing for the maximum margins of error, if the children’s absences are still within the limits, then they should be okay.