HS and TSL in September

I’ve just started planning my trip to WDW for Sept 23 - Sept 27. It will be me, DH, my 70 year old mother, my 4-year old DS and my younger DS who will be 1 year old at the time of our trip.
My mom and I have been many times and mom’s just along for the ride, she’ll go with the flow, stay with the kids if we want to do other attractions, etc. This will be my 4 year old’s 3rd trip and my then 1 year old’s first. My 4 year old LOVED TSMM the last time we went but TSL was not open yet. I’m considering getting DH and DS4 FPP for SDD and my mom and myself FPP for TSMM- that way my DS4 can use my mom’s FPP to do both.
She won’t care if she misses TSMM and I won’t care if I miss SDD… but is there a better way to do this? Someone will have to sit out both rides with my DS1 - should I try for RS?

Why can’t your 1 year old do TSMM?