We are off to the canaries in a few weeks, we are taking our just turned 3 yr old and our will be 7 mth old.
In relation to formula, we use SMA which isn't available over there is we are taking 2 tubs in the case and getting 3 or 4 boxes of the ready made (6 bottles in each box).
We are also taking a travel kettle so we can make up feeds, we have a fridge in the room so that will help.
In relation to Sterilising Asda sell Sterilising bags, they are £5 for 5 but each one can be used 20 times. The Asda ones you add water and microwave them (normally a microwave available in the restaurant). But when I was googling I did see other brands that you could use a sterilising tablet, you could also cold water sterilise in your hotel sink.
We went to Scotland for a night not so long back, long story short kids weren't meant to be coming, but they ended up doing at the last minute, as we couldn't properly prepare, we used the hotel kettle for sterilising, boiling water in a clean sink and left for a bit, it worked for us for a short period.
For your toddler, loads of mags, the CBeebies type, a tablet, a straw cup for taking off and landing to help prevent ear popping, last time we went we got a nuby one from Aldi baby event, nappies and spare clothes, if potty trained I would still put a pull up or something on as plane toilets can be a bit daunting for a little one, also a blanket so they can get comfy on the plane.
We flew Thomas Cook last time and the pushchair went right to the gate, we also got given priority security with the pushchair. This time we have bought a secondhand joie twin aire pushchair for them both, as even though my 3 year old walks everywhere, it will save his legs while walking around resort, especially at night time. We are also taking a carrier for the youngest.
You can take formula/breast milk/baby food through security, and there isn't a 100ml limit on it, I did check with our airport as it was a bit ambiguous online, you can normally do a click and collect outside if you have a boots.
Sorry for the long winded post, I kept remembering different things as typing!
Hope you have a fab time!