A previous thread of mine managed to move rapidly from urine infection to hamsters to gerbils! Anyway, the urine infection has been dealt with and the gerbils arrive on Saturday.
I'm going to keep them in a glass aquarium which was previously used for fish, surprise surprise! The lid is made of plastic and quite loose and easily knocked off. I'm looking for ideas on how to make it much more secure as both my son and the cat are excited by the prospect of small rodents.
One option I thought of might be to extend the lid downward, at the corners, so that it has to lifted off vertically from above and couldn't be dislodged by the cat.
Any other ideas most welcome....

Loose fitting lids and gerbils not a good idea.I've had gerbils and hamsters escape because the lid was loose.There may be a lids at the pet store that would fit the tank you have.They usually have clamps to keep them tight.Also the gerbils need to chew on things so make sure they have chew sticks.In a cage they chew on the bars but in a tank they need something else.Enjoy!
I'm going to put a peat/sawdust mixture in the tank so they can get a tunnel network going and provide them with loads of interesting things to chew and shred. I've had gerbils before and they're fascinating wee things, with great personalities. I love 'em! 
I'll measure the tank and check out local pet shops on-line to see what they have by way of super-security!
Thanks guys, for that!

Tigger's idea sounds great and along the lines I'd started going myself. Should be pretty simple to put together and make it too difficult for either the cat or my son to get the lid off - even if they team up!
Liked your comments GuessWho, that was funny!
I had a friend whose cat could open the front door, which became pretty embarassing when they moved to a different house in the same street and the cat kept opening the door and wandering into the old house! In the end the new people had to get a whole new door as the thing came as a unit!
Citronella oil...it won't hurt your gerbil and cat's hate the smell
Citronella oil...it won't hurt your gerbil and cat's hate the smell
That's a brilliant idea! Even if the cat can't reach the gerbils, it'd probably freak them out to have her staring ravenously at them all the time.
Thanks. 
your graphics are impressive (the pooping one was hysterical). Do you create them or find them?
Suggest you not rely on any gadget, latch, or clever gimmick -- especially since you mention a young boy.
Rather, use a 1-1/2" hole saw to perforate a sheet of 1/2" plywood. If you're feeling craftsmanlike, route a groove so that it sets nicely on the tank. Cut many of these holes.
Then, secure the plywood with one or two concrete blocks. If they're almost too heavy to lift, your son will not be able to pick them up until, hopefully, he's old enough not to eat the gerbils.
Good luck!
Well, a lid has been made - along the lines of Tigger's suggestion, although not so attractive! DIY is not my strong point! Anyway, it should do the trick and prevent anyone from eating the gerbils! Thanks, Tigger, for pointing out that it was the cat's appetite that concerned me, not my son's!

Oh, dear.

Turns out one of our three supposedly female gerbils is a male! I anticipate many more gerbils! Aaaaargh!
Oh, dear.

Turns out one of our three supposedly female gerbils is a male! I anticipate many more gerbils! Aaaaargh!
Congratulations!!!Does this mean you're a grandma?lol
Lol!
Not yet, but we're going to have to start calling Rose gerbil, Ross gerbil! 