How small? When camping it's not uncommon to have a 'pup' tent for kids. It would need to be max a two man dome tent and not tall enough to stand up in. It doesn't matter if he's 20, my son is 19 and I went to Baysbrown Campsite in the Langdale Valley where they were happy enough. There's no formal set out pitches in Baysbrown, it's just a big field. I paid £15 a night for this set up.
Check first with the owners. I have seen loads of the SMALL dome tents on pitches in the uk and France. I would emphasise the fact that it is small and that you aren't flouting the maximum number of persons on the pitch, usually 6.
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