When you need a half hour or so to teach a child one on one here's away that I use to keep a younger(1-4) child occupied.
what you'll need is a bag or box(shoebox)
some kind of toys, candy (dollar store) or rocks, something the child loves playing with
Put or wrap up the toy/candy/? in the bag or the shoes box.
Tape it up so that it will be hard for child to get open it for a while.
Now when it's time for you to teach your older child give the present to the younger child and let him/her try to tear it open! Usually by the time its opened you will be done with the instruction of the older child.