With KLM you'll most likely fly a 777 from AMS to CPT. Not going to lie, those seats are tight.

However, you're flying with children. Shoulder width is the biggest problem, and mixing the adults with the kids solves that problem quite OK in my book. You could look into the alternatives, but e.g. Emirates will most likely put you on a 777 as well from DXB to CPT, so you gain nothing in seat width.

Traveling with children, I find the 777 survivable on KLM (stuck in between 2 adults, it's a different story). Ask the Flight Attendants for an extra blanket to sit on if you find the seats hard.