I think it depends on the kids and on how much time you plan to spend in the trailer. I have three kids, and find that the 30' twin I have is about right. It allows for two separate seating areas (the gaucho and the dinette), and the separate bedroom with twin beds gives some flexibility. It can serve as a quiet place for naps for younger kids, or for kids going to bed earlier than the rest of the family, in addition to being private space for the adults.
The trailer seems smallest on those trips when we had planned an outdoor trip and wind and rain prevent us from following the plan. Inevitably there are going to be some of those unless you always plan your trips to a location with indoor activities or are willing to cancel or shorten a trip based on weather.
On the other hand on trips where the weather is perfect and everyone is outside you only need sleeping space, the galley, and the washroom, and all kinds of allowances can be made.