Wow, I want to advise against calling the cops on your own family! It’s one thing if they were abusing your kids in a violent or sexual fashion, but based on what you’ve said, it sounds to me that they are just being over-indulgent and negligent. Certainly bad things, but unless Grandpa was showing the porn to your kids, calling in the cops seems like a bad move. After all:
1 Corinthians 6
6 Suppose one of you wants to bring a charge against another believer. Should you take it to the ungodly to be judged? Why not take it to God’s people?
2 Don’t you know that God’s people will judge the world? And if you are going to judge the world, aren’t you able to judge small cases? 3 Don’t you know that we will judge angels? Then we should be able to judge the things of this life even more!
4 So if you want to press charges in matters like that, appoint as judges members of the church who aren’t very important! 5 I say this to shame you. Is it possible that no one among you is wise enough to judge matters between believers? 6 Instead, one believer goes to court against another. And this happens in front of unbelievers!
7 The very fact that you take another believer to court means you have lost the battle already. Why not be treated wrongly? Why not be cheated? 8 Instead, you yourselves cheat and do wrong. And you do it to your brothers and sisters.
Let me laud the OP for asking for help on a Catholic forum before involving the people of this world, and steer her away from the advice of going to worldly authorities instead of handling her own household. (I say this not to shame the OP, but others who were so quick to accuse).
If the Parents in law are not just negligently permitting, but actually actively showing your children pornography, that is a different case, and perhaps such extreme measures are necessary. But my plain reading of the post indicates that the boys are using their grandparents’ over-indulgence and negligence as an opportunity, rather the grandparents actually sharing porn with them.
Some good advice has been given already: your husband should absolutely handle his own folks, just as you would likely handle your own. They are more likely to listen to their son than his wife, whom they perhaps characterize in an unflattering manner. And if you haven’t already, make sure you’ve discussed this with your husband and you’re both on the same page.
If that’s the case, ask him to be sure your in-laws (his parents) are aware of why these things are important - i.e., it is not just you being controlling, but a concern for their bodily well-being (cavities) and the well-being of their mind and soul (finding porn when unattended online). If necessary, you (i.e. your husband, with your input) may find a compromise (maybe the kids can drink fruity tea or another sweet but not sugary drink?).
If no compromise is possible, then chefmomster2 has, I think, laid down excellent advice for handling the dialogue that should be had with your in-laws (as well as the discontinued dialogue after setting the initial ground rules), and also for the treatment of your kids. There is little enough you can do to control your in-laws, but you can and SHOULD lay down the law with your own children. If grounding is effective, use that. If you need to find other ways to effectively discipline your kids (after school activities, summer camps, revoke allowance, whatever) then use those instead. Your kids are the most important thing in the entire issue, the reason for there even being a problem, and the easiest thing for you, personally, to manage.
Good luck! Pray for guidance - don’t take anyone’s advice without praying over it first, not mine or anyone’s! It’s obvious you care, that you want what’s best for your kids and your family, that you don’t want to have a rift in your family over an important issue. Pray, be respectful, be loving, and do what needs to be done thoughtfully.