Oh also I'd suggest a 4 wheeled suitcase - one hand on push chair then suitcase next to you. Easier with a 4 wheel than a 2 😊
Definitely the sun block tops and hat for pool.
Poncho towel for little one. Factor 50 sun cream (a spray or roll on might be easier) A swimsuit or 2 with UV protection. Swimming sun hat and normal sun hat/sun glasses Arm bands or equivalent for the pool. Then I would just go for light weight clothing - dresses/shorts T-shirt pair of crocs. Entertainment for the plane (we had a busy board but in book form - had a delay on our flight and this kept her entertained for hours!)
Baby powder for getting sand off skin! Game changer! X
I would get one of those trolleys for suitcases at the airport and out everything on (including child) so yiu only have to push one thing.
I have a graco umbrella folding pushchair that I got in the baby sale at aldi last year for about £40 I think but you might be able to get one on market place for even less! We will be using that when we go away in April. We've not been abroad since he was a baby though so can't really help with must haves... we plan to take a few new toys/books plus some old faves so things are familiar to him. Lots of factor 50 suncream. Vest float. Pool toys. Tablet for the plane. And snacks. Lots of snacks 😂