This DIY does not get any easier. You can even include your kids in this for hands on learning of measurement skills and just having fun.
( Many commercial brands are made with chlorine bleach)
I mix 1/2 cup of baking soda, 1/2 cup of borax, 2-3 drops of Young Living Lemon Essential Oil. Place into a glass jar or stainless steel container and shake. For extra scrubbing power just add table salt to your mix.
It really is that easy! Maybe you can even get your kids to scrub the tub. No worries for safety since it is chemical free and non-toxic.