Very few fields are routinely accessible.
I have taken girls to the following:
Oundle Road, Elton, PE8 6
From Peterborough turn right through open field gate at
52.534792537283636, -0.38213105085948884
drive along track (well drained, little chance of getting stuck) for 150m where your car can't be seen from the main road.
entrance to wood on RHS. I did the deed here.
This is a bridle path but relatively discreet, easy to get to.