We gave my elder son a tub of icecream and a spoon and that helped for a while. And then by the time he was 4 he liked going to the big fancy barbers with his dad and getting the full works and treated like a big boy. It appears he just didnt like women cutting his hair

He loves it now.
The other, he’s 2, and he will not cooperate at all and does not respond to ice cream, haribo, iPads or other form of bribery (incorruptible!) so needs whatever works as simply, safely and quickly as possible. We use clippers and just crop it short all over using a vacuum to catch the hairs as he hates those. Thinks they’re spiders. He likes holding the vacuum which helps. We tell him well in advance what we’re going to do. Then remind him up to the moment. Then again in the shop, the chair. Ask him what he’d like for his hair. Show him the clippers. Let him hold the clippers. And then go for it. If there’s any flapping I hold him in a big hug with my hands over his ears or hold his hands which seems to keep him calmer as he can’t see and he feels like I’m protecting him from the evil clippers. We cut what we can, when we can get to it. No fancy hairstyles here. Barber or husband usually does the deed. Other times I’ve managed to set him in front of me watching TV, stroking his head and then clipping and cutting in small quick batches as I stroke. Sometimes he can’t tell I’m doing it. Then we tell him thank you and show him in the mirror and make a lot of fuss over how good he looks. How cool he is. What a big boy he is. How brave he was etc. So he LOVES it when it’s cut and styled and will show it off and immediately stops yelling once it’s done. He also frequently talks to me about having his hair cut or pretends to cut mine so he knows what happens and why but it’s just as soon as he sits in that chair he forgets

I give him maybe another 6 months and he’ll be like his brother I think. I’ve spoken to so many hair dressers and barbers about it over the years and they all say the same, some kids love it, some don’t. You just got to do what works for you.
Another idea is letting them see their friends, relatives or siblings have their hair cut or to help cut their hair. They LOVE that.
The other option is just let it grow until school which saves all the stress but I unfortunately I do know lots of parents who did this but are now struggling with much older children (5,6yo) who won’t cut their hair still and it’s now compulsory at our school. It’s almost like a baptism of fire. Sometimes you just need to do it until they get used to it.
My kids used to all scream blue murder getting their nappies changed or putting their clothes on too so maybe I’m just used to it?
It’s a personal choice though, wouldn’t judge anyone for their choices on this. Kids are not straightforward x