In addition to the already good recommendations above I have just started looking at Arcworlde and I think it's pretty simple with 1 = auto fail, 6= auto success. Most rolls need 4+ to succeed. Has modifiers so there is an element of math (a good thing). It's got a magic system you could include or ignore.
Rulebook is a free download, as are many of the model stats.
As much as I like the character to Alex Huntley's sculpts, no requirement to use those models. 30mm bases are technically a requirement but again could be overlooked
The game apparently scales from small warband skirmish to much bigger battles. Can't vouch for any of this though, I play my first game tomorrow
My biggest piece of advice though would be don't invest too much into whatever system. Kids are fickle
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