Sometimes certain cleaning solutions are not to be mixed.
Chemical cleaners are not always safe for pets. When you use a drain cleaner, you want to make sure you rinse wherever it was with water and make sure that it all gets cleaned up properly.
That means it's generally better to pay attention to this so you can stretch out a product and so that you know you're using the recommended concentration of the solution.
Drains can easily get clogged and cost you money when you home floods if you don't take care of them regularly.
Even if you think your child is wise enough not to mess with this kind of thing, you never know what they may decide to do or if they have friends with them that may not know anything about chemicals.
When you go with a name brand chemical solution, you can bet that it's probably going to work better and be worth the extra money.