There are lots of new players who have never played anything like an RPG before. That is the reality. I work with tons of them. A lot of them are trying D&D because they've heard about it, and a significant number are looking at it as a way to meet people and make friends. Some of them have learning challenges of various sorts. There's a lot going on and each kid has a unique story. This is not about "intelligence" and I really wish you'd stop trying to make it like kids who can't immediately suss out which sub-class they'd like from the first moment they play D&D are dumb.
For the first game, you don't want to spend hours just explaining basic rules and class/subclass options (and you don't have hours for an after school club). You want to get them playing so they pick up rules as they going along. A brand new kid to the game is still trying to wrap their head around all the race and class options without adding in 48 or so sub-classes to the mix.
So yeah, you want to keep it simple to start. If there are kids who have some experience and are picking things up fast then there are options like cleric, druid, wizard, bard at Level 1 that are plenty complicated without a sub-class. Keeping sub-classes at 3rd level give a little breathing room and costs the game almost nothing - it's very easy for those veterans who want to jump right into a sub-class to skip over those first 3 sessions and start at level 3. Or, heaven forbid, play 3 games before choosing the sub-class.
Excuse me, but if you read my post before, I said that lots of people underestimate new players intelligence and willingness to learn, but not me.
And with most of new players that was my experience, with few only not wanting to put time into learning the game and being there just "for the lulz" and because their friends are playing it.
my good friend was that in one campaign, he is very intelligent guy, but he simply didn't put much effort in playing the game or learning it.
From 3rd to 6th level(9 or 10 sessions) he was "just there".
Lowest common denominator are people not willing to learn the game for real. Met few of them, luckily in minority.
All the new players that like the game will learn it. Yes, first session or two they will suck at it, but that is the process of learning, you need to suck at something first before you can be good at it.
Give them option to swap spells/feats/skills/subclasses after every session if needed, it's even better way to learn.
Even if they want to make completely new character after two sessions, it's ok to let them.