TThe equipment/cabinet is easy. Get a cupboard lock and fit. Will cost about £3.
The tank is another matter. Yes if you have a lid that has a top opening you can get another cupboard lock for that but then you would need to waterproof it on the inside. That or put a padlock and fitting on the outside which wouldn't look too nice.
However What should be addressed is why it is a worry about a child's crawling feet becoming a problem around athe fishtank.
It is a problem that I nearly got divorced over (see following pictures) due to my wife having television goggles plus a telephone addiction and not supervising the children properly. The satellite TV was promptly cancelled by me!!!
I know they can't be watched 100% of the time but it shouldn't be such a problem that they have time to get up to the top of the tank and put anything in it.
If that is a problem then the fish tank must go!!! Forget the child putting something in the top that will kill fish yes and upset you but no harm in general to the child.
The main concern would be that if they can reach the top of the tank without being stopped that they are a few mm of glass and a huge weight of water from being soaked and possibly lacerated by broken glass. Forget the material damage to the house/carpets/flooring etc. The child could be injured pretty badly.
Silicon is pretty stron. It has to be to hold the weight of water eithin the tank but I would not like to add the kind of force even a small child climbing (holding the top rim) to the water weight.
As I say I nearly got divorced over this issue. Not because of the damage to the aquascape and death of fish but because my wife was not supervising the children properly. I am not suggesting that you aren't supervising the children properly. Just that it si a concern that should have already been addressed i.e. childproof latches on cupboards. Plug covers etc. It is something that most people have already addressed by the time the child is at crawling stage although I don't blame anyone for taking caution.
My wife had called me on my morning break at work to inform me that my eldest (3 years old) had poured washing up liquid into the tank and all the fish were dead. Not even a thought crossing her mind that it was her fault for not watching them, just that it was my fault for not having a lock on the tank lid!!
Needless to say (due to the current employment environment) I am now a househusband and no such things happen!!! The cupboard is locked yes but that has the filter, heater, chemicals including Formaldehyde and therefore I don't want them even being able to 'take a peek'. Add to this I am now open topped too!!!
Occasionally one of them (now 4 and 5) gets too close for comfort and is immediately told to move away. By that I don't mean they are looking at the fish with their hands and face against the glass. That isn't a problem to me. What I mean is one of them about to try and drop a toy or other objkect into the tank.
Moral of the story? If anything happens there is only one person to blame. precautions can be taken but that doesn't mean that your guard can be lowered
From this:
Take care and sorry for the rant. I have strong feelings on this issue
AC