How to evolve hydrogen from aluminum when the household drain cleaner is safe to use on aluminum?
First, any drain cleaner that is at all effective is probably not safe for a child to use, even under adult supervision.
Second, you can easily produce enough hydrogen to inflate a plastic bag, perhaps one to four liters, using electrolysis. Unlike an elastic balloon, the bag will not resist inflation appreciably until full. Try carbon electrodes (e.g., from zinc-carbon cells) with $\ce{MgSO4}$ (Epsom salt) in water as the electrolyte.
You can also capture oxygen generated and test it with a glowing wooden splint.
It is an exercise for the experimenter to calculate how many ampere-hours it would take to inflate the specific bag.
BTW, you might want to perform this outdoors, in case a spark sets off the hydrogen!