Since vinyl is a lower-risk building material than cedar, it is more likely that your homeowners premium would reduce rather than increase. For more information, call your insurance company’s customer service line to see if they will adjust premiums for the building change.
Your son would not need to be listed in any way on the policy unless he is a part-owner. Any payouts from insurance would go to the lender and homeowner(s), so having him on there would not do anything for him.
If it was not disclosed to your insurer at the time of finalizing your policy that you're running a business out of your home, it is within the company's right to void it. While you likely won't need a home daycare endorsement if you already have separate liability insurance, every potential home insurance company will want to know that your home is also a place of business.
O and L stands for Ordinance and Law. Though your agent said that it wouldn't cover code upgrades, perhaps he meant that if a code went into law, your insurance isn't going to accept claims to upgrade the home and bring it up to code — unless your home suffers a loss.
What kind of insurance you need depends on a few factors of your rental. For insurance, a million in liability may sound like a lot but for most businesses, this is just a starting point because the risk of lawsuits and other exposures are so much higher.