If you're buying chemicals from the store, you're going to want to read through the warnings on the packaging to see what it says about pets.
You don't want to use something like this until you understand it better.
If you're going to be using chemicals in a bathroom or somewhere without windows, you need to make sure there are at least some vents in that room that can help the fresh air to circulate.
If you don't want to have to study up on what you cannot mix with what, you may want to find a plumber to deal with it for you instead.
When you're going to use a drain cleaner from the store, you need to know that they sometimes are not a good fit for your drain system.
You always want to assume that a child doesn't care about the rules, just to be safe.