I'd do both, but with young kids, I think they would like the African Lion Safari better! Toronto Zoo is huge, be prepared for a LOT of walking! Many exotic exhibits with enclosures replicating their natural habitat, but again, a lot of walking! Your young kids might get a bit impatient and tired after spending a day there, I know mine did!
African Lion Safari can be very appealing to kids, with a lot of interesting exhibits/shows to captivate their attention, the walking portions of the park are not too big. Take their bus through the safari portion, not your car, the monkeys are notorious for doing damage to your wipers and any other protrusions from your car!
After seeing the animals, there is a nice waterpark at the Safari that I know your kids would enjoy on a hot day, mine certainly did, I had to drag them away!
Just don't let them lure you under the tipping bucket!
