I do question the wisdom of travelling with two small *unvaccinated* children and a heart condition during an out-of-control pandemic of a disease which causes massive blood clots. To a state with a case rate over twice as high (93/100K/day) as the OP's home state (39/100k/day).
If I were the OP, that's what I'd be worried about doing, and I wouldn't do this trip. I think the trip would be perfectly fine *at some other time*.
I have seen many moms with young kids travelling in coach seats quite happily. I guess having other passengers nearby might give more reassurance than being alone in a roomette?
At what age does a youngster get a seat of their own? I assume up to a certain age kids travel free, but don't get a seat?
I am sure that you will be fine on Amtrak, don't overthink it, it is a routine harmless event for 99.99% of passengers!