Did you try the booking online? If so, try calling them instead. I'd think they'd allow two kids that young to travel in the same roomette with you.
When I tired booking the 3 of us online it didnt even show a roomette or bedroom available. Just coach seats. I did call, and they said that a roomette would be too small. I would have to get a bedroom. I am just wondering if I book the roomette for me and one child, and the 2nd kid as an adult if I would be allowed to bring her into the bedroom with us, or even allowed to board if one child is assigned a seat and the other one is booked into the roomette with me. Thanks so much for any input.
I like the idea of getting 2 roomettes. If your traveling dates are flexible, check out AmSnag and see what kind of deal you can find for the two. http://biketrain.net/amsnag/amSnag.php
p.s. If you are not familiar with AmSnag, and I just realized you said this is your first train trip, and are probably not, someone here can help.