I snagged Mentzer's red book last night, thanks, guys!
I haven't decided against using D&D, I just bastardized and simplified it to kingdom come.
My system is more or less D&D but without any of the hassle. Just humans, dwarves and elves. Single D20 roll for all tasks, roll under your attribute to succeed. Just 3 classes (Fighter, Rogue, Wizard) and 4 attribs, one attrib for each class and CON for all resist and endurance checks, poison, hunger, swimming, etc. One universal "magic" class so no need for WIS. CHA was always pointless in early DnD anyway. No clerics because religion in RPGs is not a great idea with children, I feel.
I stole all monster skills and flavor from 5E but an ochre jelly looks like this:
Skill: 12 (roll under that to hit players or roll under when monster is making a saving throw)
Armor: 1 (when rolling under your STR for melee hit, rolls up to enemy armor value are misses, so enemy armor is baked into the player's attack roll, while the player is still rolling against their own attrib)
Health: 7 - 11 (at DM's discretion)
Damage: 1d6
Immunity: Immune to lightning and bladed weapons.
Amorphous: Cannot be critted or backstabbed.
Splitting: Lightning or blade damage causes it to split in two, each at half of its original HP.