If you refer to the fare charts here...
Long Distance Train Coach & Sleeper Fares (Buckets) ...you'll notice that it's possible two Roomettes could be cheaper than one Family Bedroom for the three of you provided you have some flexibility in your travel date.
In addition, consider that the Family Bedroom windows are only half the width of those in other sleeper rooms:
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And if this is a one-time occasion, uh, what's money got to do with it anyway?
I'd go for two Roomettes across from each other and to get them you'd need to make the reservation with an agent - can't get that arrangement by booking online. If you try booking two Roomettes online you might even get Roomettes in two different cars!