I can put a lid on the barrels, but is there something you can put in the
water to prevent mosquito breeding in something large like a wading pool?
Tom,
How about getting some 'mosquito fish'? There are North American native
species of Gambusia minnows, which really are nothing more than a hardier
variety of the ubiquitous pet shop guppies. If you don't have a ready
source of the native species, perhaps you could just go to the local pet
store and purchase some guppies.
Gambusia minnows love to eat mosquito larvae, and you wouldn't have to
worry about any potential toxins, etc. that might harm your plants - and
the ammonia from fishes' wastes would be of perhaps a slight benefit for
any plants you used the water on.