i usually wear jeans and converse, my children will stand on my feet so sandals are a no no. I take leggings and a top in my hand luggage for myself aswell as a change of clothes for the kids, quite often they manage to spill things on me, or my dh doesn't put the nappies on correctly and they leak on my lap.
Will you need pillows or blankets? we travelled with easy jet recently and they don't have them. enjoy your hol.
By the way if you wear a belt you will need to take it off when going through security, more hassle when trying to control three little ones and hold the baby.
What about cotton leggings (jeans will feel v hot when you get there...) and a shirt style tunic like this (should help with bf?) teamed with cute ballet flats (I wouldnt wear lace up shoes, they often ask you to remove shoes at security now)
Camisole/vest plus cardi or wrap top (Boden has some jersey ones for winter that would work but may be too late if you are flying on Tues). for breastfeeding (and cardi will keep you warm on plane). Agree with Gillybobs that jeans too hot especially if you have to hang around at all. How about chinos or long (not maxi but sort of stylish above ankle length) floaty cotton skirt (thinking of the Toast ones) and ballet flats or espadrilles (easy to take off, comfy and OK if your feet swell).
Last year I wore a maxi dress with a cotton cardigan over it and took a pashmina and socks to wrap up in on the plane. I'd far rather be a bit cold in England than arrive at the other end and be boiling.