The message
from Sacha contains these words:

I suggested that the OP told the children that they could go into his garden
at any time - in other words, that they have his blanket permission to
retrieve the ball and do not have to keep ringing his door bell and asking
his permission.

This does have dangers. It would be wise to include the concept of
'accidental' into the equation, and 'until or unless I say otherwise' -
with such open-ended blanket permission there is a possibility of
generating a *LOT* of bad feeling, and redress might be very difficult.

--
Rusty
Open the creaking gate to make a horrid.squeak, then lower the foobar.
http://www.users.zetnet.co.uk/hi-fi/