First thing to say: relax, hopefully it will all be a lot more straightforward than you fear
We have done 20+ flights since our 2 year-old was 3 months, and these are some top tips:
*Always take more nappies and changes of clothes than you expect you need. Bear in mind possible delays, as well.
*Take a spare top at least that will do for you/your partner in case of poonami. We've never had to put that to the test, but a friend recommended it.
*Good if baby can be drinking/sucking on take-off and landing to help with pressure change.
*Pampers are called Dodot in Spain. Huggies are Huggies and Johnson's stuff is Johnson's...!
*I don't really have anything useful to contribute on the milk question, but I'd be surprised if you were made to open sealed baby food. I've always got yoghurts and compotes through with no problem. Always have to sip the water, though...
*You can buy stuff in Boots after security most places. And many of them you can pre-order baby stuff if required.
*Someone has definitely said to me that formula is more expensive on the continent. To get the brand you want anyway it might be best to take it with you anyway? Others will be able to advise what to do re: sterilising and water. I really don't have a clue what I'm talking about, but people had mentioned to me about microwaveable sterilising bags, which might be easier than taking a steriliser, if you have access to a kitchen?
*Always good to take a sling if at all possible. You usually can choose to check in your buggy with your suitcase or take it to the gate (we've lobbed our Bee+ in the hold everytime with no protective bag with no issues - with the hood and any liner/footmuff/raincover in the luggage. Worth pointing out, if you plan to try it with a similar buggy, that you need a tag for the seat and the chassis. Especially if travelling to Madrid
) You get your buggy back at the bottom of the stairs when disembarking the plane or, more commonly, on the luggage carousel/weirdly shaped luggage section. So potentially a fair bit of walking, and useful to have both hands when wielding passport, changing bag etc.
Have a wonderful break!