How do I put parental control on ads in my children’s apps?

I’m trying to find a solution for the ads that continuously pop up in the games my kids are playing. I haven’t found any control settings for this. The games are for 4-6 year olds. Then the ads suddenly pop up with little cartoon things killing each other. There’s so many games like this. I have to sit over my child and make him hand me the iPad every time an ad pops up. There should be no knives, guns, blood.... etc imbedded into children’s apps! How do I fix this?