Dragon Rampant is an easy to play ruleset for 4-10 units of fantasy warriors from any maker by Daniel Mersey based on his Lion Rampant rules for medieval gaming. Both available from Osprey.
https://ospreypublishing.com/dragon-ram ... osprey_usaThe * Rampant series of rules have been the most inspiring set of rules I've come across since Blood Bowl 3rd edition in the early 90s. I love them and they've rekindled my desire to do fantasy battles. They're written very clearly with a humorous and joyful approach.
I highly recommend Dragon Rampant and hope you and the kids get to enjoy it. My own son is 15 (I'm 47 and I too played Heroquest as a kid) and I'm hoping to get plenty of games with him in the coming years before he moves out.
EDIT: One of my goals with getting into Dragon Rampant is to do something that's fast-to-the-table and low cost. This ruleset and 1:72 scale make both possible and the fact that the rules are great is just icing.