I think it's an excellent idea what you did with your children because they learned an important lesson while having fun as a family. Although I do not know if the guys hold a full game of Monopoly since the game is very long.
If you had taken the time to explain them verbally or with a blackboard, they would surely have been bored.
A question. Were they aware that these loans violate the rules of the game?
Haha. I never told them the loans were off-limits. I think that rule exists to protect people from falling prey to (high interest) predatory loan situations, a reason which I understand. But when I take an equity interest in a newly built house, I am not taking any interest payments; the player with the property still gets more than he/she otherwise would and I only get paid when he/she also makes money from someone landing there.