Yes 20 an hour is a good rate for two children plus what you are doing with them a day. You are a super human!
Some activities I've always thought were a good idea were hands on activities. If they have learning toys that make sounds then play with those. Also even though the baby is I=only five months you can do art activities with him/her and give it to the parents.
The best way in my opinion is to speak to them in a calm and mannerly way. I have a 3+ year old niece who cooperates better when you don't raise your voice at her. Be as calm but firm as possible.