before we moved from austin to waterbury last year, we had done this before ourselves. the train is rarely even close to full on #22 northbound from austin, and yes, that's even when there is a lot of traffic. although there will be a fair number of students boarding during spring break, you'd be surprised how many people superliner coaches hold. you shouldn't have any problem -- we certainly never did. as for boarding on weekends, you board from the austin station; there is usually a volunteer who opens the waiting room, even though the station isn't officially 'staffed' on those days. if he/she doesn't show -- they always did when we boarded on weekends -- there is a covered overhang outside, or you can just wait in your car in the parking lot. the station itself isn't too much to crow about, but at least austin has train service.
as for being on the train, you're kids will probably love it. our two-year-old certainly did, and given little impressive scenery between austin and points north, the lounge is rarely full. the children usually enjoy seeing the countryside run by outside the huge windows, and if you have a recently refurbished lounge, there are tables upstairs for them to spread out (there are tables downstairs regardless). if you're an austin native, your kids will probably get a kick of traveling up the middle of MOPAC. take a few toys with you, but with the short trip into fort worth, it should be a fairly quick 4.5 hours. if the cost doesn't bother you, the children will probably also enjoy lunch in the dinner. i'm not sure i'd spring personally for a sleeper compartment: with more than one child a roomette will be very cramped, certainly more so than coach; a bedroom is a little pricey for the trip up to fort worth -- but, again, if cost isn't a factor, you may appreciate the privacy.
the train itself is very safe, and once you get your 'train legs' and the kids settle down, any trepidation about the trip will melt away. enjoy the trip, and let us know how it went!
-- eliyahu
waterbury, ct