Kinda hard to picture, but in a nutshell you want to close a gap in the
fence using plants?
Well Bouganvillia or 2 would be good, and isn't too invasive if kept pruned.
Only problem is all this rubbish with sueing and what not now days, is if a
kid tears themselves up on them, are you liable. Commonsense would say no,
but overpayed doo gooding solicitors would say yes (ducks and says not all,
before flames come in).
Otherwise, you could go the Grevillea that has the spiny short "pine like
leaves". Can't think of the name, but ask at a nursery.
They are quite itchy when touched, and annoying.
Whatever you decide on, especially the grevillea must be relatively
advanced, say at least 2 foot high to start, otherwise they will probably
just get trampled.
Good luck