We went to Spain in May.
Alex was 8 months old, and Lydia was 2 years 7 months.
Let me reassure you that it went pretty smoothly. Alex boobie-fed most of the way through and that helped him to sleep through the landing.
As for Lydia, we had pre-prepared her for what was going to happen during landing and takeoff, and as a result she wasn't scared at all, just excited. She sat very well all the way through the flight and looked out of the window, and we took a few books for her to look at as well.
Alex was on my knee - the air-stewardess gave me a special seatbelt that went through my belt and around his waist. They showed me what to do with it. It's not difficult.
At the back of the plane, one of the toilets has a little changing table that you flip down to change a baby on. It's very small and doesn't have a safety belt for the baby or anything, but I didn't find it difficult to use at all. I mean, the toilets are that teeny that he couldn't have fallen anywhere anyway as I was taking up the whole toilet lol.
Oh, and you are allowed to keep your pram right up until you get onto the plane. When you are at the stairs of the plane, you just take baby out of pram and fold it up and hand it to one of the baggage handlers. Then when you get off the plane, just ask one of the staff to get it for you at the stairs again.
The only slight problem was that once we had gone through the boarding gate, just before reaching the plane, there were a lot of stairs to get out of the airport and outside to where the plane was. It was difficult to push the pram down the stairs whilst juggling hand-luggage plus Lydia. Some other passengers grabbed the bottom of the pram for me though and helped me down the stairs.
We actually had two prams with us when we set off - one for Lydia and one for Alex. But we decided whilst we were on holiday to leave one of the prams in the hotel room (it had only cost £15) because we didn't want to have to handle both prams on the journey back. It was much easier to have Lydia walking. I'm very glad we decided to do that.
I'm actually off on holiday again on Thursday with my mam and the kids. We're off to Spain. So I shall be tackling the planes again with a 1 year old and 23 month old. I'm not even slightly worried though about the flights now.
So yes, my one piece of advice is to only take 1 pram if you don't need one for Seren. Or use slings. Other than that, take books and things to keep the kids occupied. Lydia got a bit bored in the airport coming back because we had a longish wait before we boarded.
That's all, I'm sure everything will go fine for you. Have a fantastic trip