After having my son I joined all the mom and baby fit classes I could find. I absolutely loved it. I did notice that there are lots of options for mom and baby and not many for a mom and tots. I tried to go to the gym and leave him in a babysitting club. He really didn't seem to love it and I was always jumping off the treadmill to check on him. If I went alone, all I thought about was getting home to my wee man!
I have found a good balance for that toddler stage. I'd work out at home and do a lot of fun exercises where he can help or do it with me. I invested in some home gym equipment for our mom and tot work outs:
uber light kids weights so he can "work out like mom"
lots of colored bands to keep him busy. Bands are bright and he likes to wrap them around things.
Bosu Balls are great, he loves to bounce and jump on it.