Mother's Help/ Sitter Every Now & Than

We are looking for someone who is willing to be on a "on call" sitter. We have a regular Nanny that works for us during the week, however, there are times where she (can't work/requested days off) and we need help filling in. It would also be nice if this person was able to work on weekends for a couple hours during the evening if my husband and I wanted to go out and/or if this person wanted to travel with our family altogether. We are very flexible, just looking for additional help. The only child you would be looking after is my 2 1/2-year-old son. He loves to be active and is the sweetest kid. I'm a stay at home mom so I'm usually around but taking care of twin babies.