I just hit mine head on!
When dd 4 was a newborn, I took dd 1&2 to gymnastics. Two hours, two consecutive classes. I was with dd3 just under 2yo and dd4. It was a nightmare. My only saving grace was that for one of the classes some mums of dds friends were there, they made the time pass quickly. So, I'd recommend doing it with friends, so that you can catch up at the same time.
My experience with gymnastics was awful because there were lots of kids whose parents were not really looking after them, and they constantly bother me. Constantly touching dd, ESP while I was bf. I'm not good at grill g them to go away, ESP as the worst protagonist had an awful grandmother who'd just yell at her the while time. I stopped that class and we now do a Saturday morning one. Once again, I meet up with a girlfriend for a coffee with our dds - makes it bare able, I sometimes have all my dds who are not in the class with me, or dh may take the baby (now 2.8) to the park.
We do swimming on a Sunday - usually swap taking them with dh. And I take the youngest during the week.
Music on Monday, five minutes walk away, I either chat to mums and dds play eith other dds or go home and put tea on!
Sensing a theme here ... I need adult company to make it bareable! Mine are all girls, and I try to steer them into the same thing. However, they are about to choose their own instruments, so it could all go to pot.
Take baby in sling, lots of snacks/milk. I used to also take books and pencils. Play Simon says, enact stories (going on a bear hunt was a favourite). It's tough, but I wouldn't deny them the experience of extra-curricular activities, so I just did it!