Mother son school party

So, my step son's school puts on a mother son party every year(to clarify, the poster does say mom, grandparent, or caregiver. So not strickly mom). It's always fallen on a mom day, but this year, we had a change in custody, and now it falls on a dad's day. The boys are 7 and 8. This year, we swapped days, so she could take them since it's something she has done every year since kindergarten. After, the 8 year old told me that instead of switching days, dad should have stayed home with our baby, and I should have taken them 🥰😭. I almost cried. He said next year if it's a daddy day, which it will be, he wants me to go with them instead of switching days. I have no idea how to handle that since she will throw a huge fit. I want so bad to go with them, especially since they asked, and as stepmoms, I'm sure we all know that feeling, but I also don't want to overstep on moms role. She is very hc so both of us attending will not work. I'm just wondering if I back off next year and explain to them that it's really special to mom to take them or if we try talking with her to make it happen for the boys. I already know it will be turned against us even if it's something the kids ask for.