We are a family of 4 (kids are about the same age) that went from a 17’ Casita to a 21’ Escape. You may need a different tow vehicle for the 21’ but I think you will really appreciate the space with growing children. This is especially true if the kids become teens and plan to keep sleeping inside the trailer and not in a tent.
When our kids were young and it rained, even the 17’ was tight because we were all on top of each other. Now when it rains, we can set up the table and sit around or lounge on the permanent bed… or do both! There’s more space overall and we can comfortably get past each other between the kitchen and the fridge. Couldn’t squeeze by the bunk beds and the stove in the Casita!
Hats off to you for camping as a family of 4 in a 13’ trailer! I don’t think I would’ve survived one family camping trip.