WHen we flew into Malaga with Easyjet last year I figured that it was only £30 mnore to buy another seat and sit our lad’s car seat, with him in it, on the plane. Saved us from having to keep him still on our knees, which you can do until they’re 2. If you’ve already got seats for the kids then I think airlines encourage you to take a car seat on board for them. You also know that the seta hasn’t been puked/sh*t on or been in an accident already.
Just check the seat measurements to see if they will fit the plane, I thought it would be touch and go looking at it but there was plenty of space.