By all accounts the culture of the studio changed quite a bit as a product of the restructuring of the early 90s. Ron and Shelley Day were not the only people to leave the company around that time. Presumably they were just ready to strike out on their own and have stake in their own creations. As far as the motive behind making adventure games aimed at children, Ron told Mojo that "it gave me a way to do more games in a short amount of time and experiment with design issues."
I think Ron saw which way the wind was blowing with adventure games in terms of growing budgets dooming them to become unsustainable, and the early Humongous years can be seen as sort of an antecedent to the digital revolution that would come. The internet was not an option yet, so games aimed at children, who didn't demand bleeding-edge graphics and 60 hours of gameplay, was sort of an alternative manifestation of the same idea Telltale popularized: make adventure games quickly and cheaply and thus profitably.
The evolution is even more striking when you look at what the short-lived Hulabee, the successor of Humongous, was attempting. Ron was openly talking about downloadable adventures games by then, but the business fell apart before he could really prove what he set out to. But you look at the six-chapter Moop and Dreadly, and it's like Ron was predicting the future. Interestingly enough, Freelance Police (which would have entered development around this time) was itself conceived as a six-chapter adventure game, and the team had lobbied for digital distribution, which was too radical for the management at the time. (Ironically, the same management ultimately cancelled the game because of the cost of bringing an adventure game to retail.)
Finally, Sam & Max: Season 1 demonstrated that this could work, but it's funny how different people were independently working toward the same conclusion in the years leading up to that.