OP Id three under 4 and travel solo with them a few times a year. Children under two can bring bottles - for milk, most airports will test it (using their machine) but this can involve breaking a seal on small liquid bottles. I find it easiest to bring powered formula decanted to the amount of scoops in the base of each bottle (add hot water and shake a lot, top with cool to make up). I tend to get water in one before getting on the plane - but they can do it for you onboard. (With Ryanair the hot water is free but you will need to buy cold bottles to top up). If traveling with breast milk it’s the same kind of set up - they’ll likely test bottles and return them to you, and heating milk is possibly (slowly) in a cup of warm water. They won’t often microwave them, as they’ve an oven not a microwave on board.
You are also allowed baby pouches and snacks (typically I’ve about one sandwich sized bag of each, per kid and it’s never raised an eyebrow). Buggy’s - depend on the airline but EU rules are two pieces of equipment per child and most airlines allow 3 for infants (under two). Ryanair allow you to bring a collapsible buggy on board if you have priority (it counts as your suitcase). If no priority or your buggy is bigger; you can still bring it to the gate (which means the steps of the plane) and it goes in the hold. I find this easiest! As I can’t manage to hold the hands of my little ones, have their change/snacks/milk bags, help them up the plane stairs and haul the buggy.
baby’s can bring their own change bag (I’ll always have a spare change of clothes for them in it in case) - separate from your allowance, but just remember you’ve to be able to carry it all - plus baby - (or ask for help).
just as an fyi - a more recent thing is that Ryanair often look to tag a buggy at check in - so this means going to baggage check. It’s also baggage check where they will tag your car seat/cot. Grab someone to help on the automatic machines as they need a staff code but everyone’s often been lovely to me.
Honestly, it sounds more stressful than it is. I know the travel I do is most people’s worst fears, but generally people are lovely and helpful. And the kids have you! So that’s really all they need.