Here are the 4 and 5m pitched together for reference
We also have our size guide video that may help.
We're a family of 3 also, we have a 2 /12 year old! We use both 4 & 5m sizes, the 4 is great for long weekends, etc, but for longer term week or above I'd probably steer you towards the 5m size.
Thinking about future proofing, the 4m is good for now, but as the tents should last a long time the extra space of the 5m will perhaps be appreciated as the little one grows!
We're based in Newark Nottingham, so if you wanted to call over, we're currently open during the week to show you around, with open weekends planned later in the season also.
Hope that helps with some guidance.