Yep, at easyjet we class a seat in front/behind (particularly aisle seat) as being sat together. It doesn't always satisfy the parent but in my book it is sat together and it would have no impact on a evacuation. Also advise parent to arrive at the gate earlier to be boarded first.
Like I've said before though, loads have to be circa 175 (320) and 151 (319) before you come across any issues.