A verbal agreement that you'd transfer the house into your son's name when it's paid off would not be legally enforceable, even if they had gone ahead and made all the payments. However, since they're not paying, they really don't have any legal claim to the house, because your obligation to transfer title isn't triggered until they finish making the payments. At this time, they're essentially your tenants because they're in the house with your consent under an agreement that they'd make the payments. And that actually means you can have them evicted. The law says that a landlord can ask a tenant to either move out or pay the amount owed in full by serving a 5 day written notice. If they don't or can't pay, you can go to the local courthouse and file forms to have them evicted on the sixth day. The courthouse should have forms you can use to open the case. This site has more specific information on the process that might help.
Now, an eviction means your grandchildren are also evicted, so if you prefer, you can get a judgment against them for the money you're owed, which includes the mortgage payment, property tax, and insurance. You're allowed to ask for up to $7,500 in Small Claims Court, but it sounds like they might owe you more than that. If it helps, you could divide the property-related cases into one lawsuit and save payment on the other debts into another. That's an option.
If you can establish that the children are in danger because their parents are not looking out for their health and safety due to their gambling addiction, then you can get the state involved to ensure that the children are cared for. But if that's not the case, I'm sorry to say, Oklahoma law allows married parents to decide on their own who can and cannot see their children. Grandparents have no legal ability to bring a lawsuit in this situation unless you can prove that they're unfit parents. That's tough - and it would be best to have a local attorney help you because the law isn't on your side. The best chance you have at seeing the grandchildren is to see if you can appeal to your son, get him to agree to do the right thing.