You haven't said what type of fencing you have, but the quickest solution would be to add rigid trellis sections on top of the fencing you already have - these come in sizes between 1 and 6 feet deep. Then plant climbers along to grow up and twine in the trellis, together maybe a couple of pyracantha grown against the trellis/fence to provide a thorny, evergreen barrier. Any shrub or plant which grows extremely rapidly will just keep on growing and not stop, usually, so there's no rapid solution from the plant point of view. Only exceptions are Eucalyptus gunii, Buddleia davidii and shrubby Mallows (Lavatera). The first would require severe pruning twice a year to keep it in bounds, but none of these plants would prevent children climbing over or even be particularly good at increasing your privacy dramatically.
Regarding the climbers over brickwork, as the brickwork is not in good condition, you cannot use ivy nor virginia creeper - providing some kind of support on the wall would enable you to grow things like clematis, passiflora, akebia, depending on the amount of sunlight available on the wall. These plants would all need pots a minimum of 2 feet deep and 12 inches wide to achieve anything like their full size in the ground.

Hello and welcome to Goy. Sorry to say this but to a certain extent if you cannot afford 2 meter high wooden fencing - you have to learn to live with the youngsters? Yes they will look/climb over if they think they can get away with it and shrubs wont stop them either.
The trellis idea is your quickest way but if its a cheap structure it will not last long if balls are constantly kicked against it as they are likely to be, and I doubt trellis will stop them climbing over for long. And as we saw in the press a couple of years ago you cannot even keep the balls!
How about a 2 meter high metal link fence most of the balls are more likely to bounce off that and stay in their garden you could grow thick ivy on it to screen etc. Its also quieter than a ball being constantly kicked against a wooden fence. Perhaps if the parents bought them a garden football net that would help too?

berberis will stop them getting into your garden - will tear them to shreds!

Some already well grown Pyracantha but could be pricey. Nice and prickly. Pretty flowers and berries for the birds. Reasonably easy to keep under control with pruners and thick gloves. Berberis in front of that if wide enough. Maybe some prickly roses too. Another thought is a lovely prickly blackberry or two. They grow quickly, if a vigorous form, and you could have lots of bramble jelly. We used to grow a very active prickly one for the fruit but can't recall the name. The growths could be trained along the top of the fence or near the top. the one we had, no one could go over it!

Any kid worth its salt will still get over a 3 foot fence, no matter what you grow over it - we used to get an old blanket and lay it on the top if there was prickly stuff when we were kids and wanted our ball back.

Thanks to everyone who responded. I like the blackberry jam idea which i could have after my home grown vegetables that are doing so nicely.
I have found some good size wooden boxes with trellis attached which will help with the brickwork. I will give all your suggestions for the fencing a great deal of thought. I have spent so much time & money on my garden hopefully i will now be able to enjoy it .
Thanks again to you all!!

Bear in mind that the blackberies will attract the kids too and they'll develop ever more devilish ideas in order to reach them!

If anything teaches todays children that fruit grows on shrubs and trees and veggies in the ground. It can only be a good thing!!!!!!! The children would be welcome to a few for the VitC and other alimentary benefits!. Bits of carpet and scrumping aside.....the blackberry we had would repell all intruders apart from those with a flame thrower.

I think a blackberry would look awful most of the year and make her main garden look scruffy and unkempt? As Linzi says she has spent time and money on her garden and wishes to enjoy it I dont think this cheap invasive option would make for a happy gardener if its in the show piece area?

Linzi, another piece of advice - the boxes with trellis attached that you mention - if they're the ones that are like troughs, bear in mind that, if they're less than a foot deep, you won't get much that'll get tall enough to actually climb up the trellis because of the lack of depth for root room. On the other hand, the trelliswork is usually attractive in its own right and may well disguise the brickwork on its own.

Blackberries don't look awful if they are well trained, Drc. You need three lengths of about 12 feet of wire at about three different heights (you'd have to have the trellis or there wouldn't be enough height.) Then you train one years growth on one side and the new growth for the following years crop on the other. As long as you are careful to train in all the new briars it looks quite neat.

We will have to agree to differ you should see mine Steragram, well actually no one sees them they just dont behave that way for me.