Well, at least you will have fertile soil for your vegies.  But I will
try to be more helpful than that.  :)  

These are still very young cats, and you ought to be able to convince
them that mother knows best.  Do not get rid of the inside litter tray,
first of all, and be sure that it is scooped clean of solid waste AT
LEAST once a day (if their is any), and that the litter is changed AT
LEAST once per week per cat, which would be twice if you have only one
box.  Many experts will tell you that you should use separate boxes for
each cat, but I've never found that to be necessary.    Keep it in a
good place that has some privacy but isn't at all difficult to access.  
Don't keep it next to their food.    Reintroduce them to the litter tray
by putting each of them in it a couple of times, if it's in a different
place from where they had it previously.  And Use a decent, dust-free
commercial litter, as well--not sand.  

When you let them out (which isn't safe to do in most places, but for
the sake of discussion I will just assume that you have no cars, no no
stray or wild animals, no sources of poison, no insect parasites, etc.,
where you live), you don't want a sand box, especially if they actually
use it.  It will become a stinky mess if you don't replace the sand
often, and those animals that you of course don't have in your area
might also use it, and might bring diseases.  

Lastly (or firstly--your call) put a large sheet of plastic over the
vegetable garden, and poke holes in it to plant your seeds or young
plants.

Others on here may have more ideas.  Good luck.  

Diana

I thought I read that scattering citris rind (oranges,
grapefruit, lemon, lime...etc) would discourage the cats from
using that area.  I believe that makes good mulch too?

Sara
too much a city girl to even think about planting a garden or
letting the kitties go outside

Everybody wants to get rid of old refrigerators, so I go to disposal sites
and beg the wire shelves from them.  I now have a huge pile of wire fridge
shelves
in my garden, and when I plant directly into the garden, seeds especially, I
cover the site with these fridge shelves until such time as the plants come
up.  Once they're up and growing your cats won't want to "go" there.  We
have two cats, and AFAIK they rarely go in my garden, and happily my
neighbour doesn't seem to mind.

someone

Catch them doing it again, and stick a firecracker up their a.ses.