The crew was correct. Anyone, no matter who it is, cannot roll around on the floor of a plane. I, as a passager would demand the mother with her chiild leave the plane, as long as it is possible, and the plane is still on the ground. That includes an unruly healthy infant who cannot be silenced, since passengers cannot leave the plane.
If I am flying, and pay a good price for the ticket, I expect comfort, it is an imposition listening to a crying infant.

I don't care if you kid is Autistic it's your job to control your child. Don't fly if you think it'll be an issue. Take a train, ride a bus, drive... It's not like you woke up one day and found out your kid has problems... Plan ahead for the safety and comfort of everyone.

Do you know what it cost to ground and purposely delay a plane for 20 minutes? It is not the airline's responsibility to sit on the ground for 20 minutes incurring cost while a passenger gets over their tantrum. As much as I have sympathy for this child and mother, rules are rules