Is SMA, (child version of ALS), rare? Statistics? Likelihood of it happening in a 3 yr old child?

Rare/ not ALS. The Spinal muscular atrophy types (1-4) arise from an inherited lack of a material that protects the motor neuron. The type 3 (onset after age 1yr) has an estimated gene prevalence of of 1/375,000 in the population and incidence of 1-6/1,000,000 births. You need a defective gene from both parents to inherit this condition.Though weakness is common to ALS/SMA they are not related.