OMG. This is one of the WORST ideas I've ever known.
Makes absolutely no difference if it's on private land. A girl was abducted from a tent in her own back garden in the UK (can't remember the full details, it was about 15-20 years ago). She was camping out with siblings/friends in a secure, private garden.
I realise abductions are incredibly rare, but they could escape, wake up scared, get absolutely soaking wet, smother themselves or each other - it's NOTHING like sharing a bedroom at home where the surroundings are familiar and they're in a proper cot/bed in a proper room.
I can't believe you'd even think of putting them in a tent on their own, and don't imagine the owners of the private land would think it a sensible idea either. Hopefully they'll stop you if we can't convince you it's a very bad idea.