My personal view is that for pre-gen characters; I'd build them so that they demonstrate the role-playing aspects and the system mechanics they need to learn so they can be more engaged with the game. I don't think there's a need to be bound by PHB options, build them like NPCs so they can focus on the play, not the character building rules.
1) I'd make one Str based melee attacker with a greataxe and a throwing weapon. (Don't go greatsword, you want them to associate "One weapon attack = one die" as a general principle). One Dex based archer/rogue with a finesse weapon backup. One cantrip user/caster (but not Eldtrich Blast or SCAG cantrips) with a backup weapon. (To demonstrate that different types of attacks use different stats.) This set-up gets them familiar with the general tropes and rules around weapon use and at-will attacks that 5e uses.
2) I would give each of them a bonus action ability, that recharges on a short rest. This shows them how bonus actions work, and displays that they are typically a "sometimes thing". You don't want them to assume that doing two things every turn is normal and expected, as plenty of 5e characters don't have good bonus actions. But they should definitely know how they work, as I've seen them be a pain point for 5e newbies. I'd give the Str warrior a "Dodge 3 times per SR ability", the archer/rogue type a "Disengage" at will plus a "Dash 3 times per SR" (rogue-types generally have better BAs, but you don't want to get into Hide rules with newbies; Disengage is also situational), and the caster a "Help at range 3/SR" ability.
3) I would give them more hit points and hit dice then their abilities would generally indicate, I'd make them around 4th level in terms of toughness. You want them to have some buffer to make mistakes, and it gives them more chances to use Hit Die, which are also a resource that sometimes confuses new players.
4) I would also give them one long rest resource. Probably a rage-type ability for the warrior, one casting of invisibility for the rogue/archer, and shatter, a healing spell, and a concentration spell (maybe bless) for the caster. Give the caster more uses (at least 3/4 slots). It's important to learn the basics of spellcasting and long rest resources, and it shows that casters have real utility to make up for the lower damage and defenses they're stuck with.
5) Finally, give them varied skill proficiencies. Make them trope-appropriate.