How do I stop inappropriate ads on iPad games?

Since the ads are coming from the developer of the App, the first thing I would do is contact them and see what their response it. Depending on how they address your concerns may help you in making the decision to support the developer by using their product. Every time an ad comes up, they are getting paid.

Apple has no control over the ads that a developer of a game displays. The developer gets money from the ads and Apple does not. You can report a problem about the App at the following link and choose the reason as Abusive Content. Apple may be able to put pressure on the developer by requiring them to increase the Age Restriction or even remove the App if enough reports come in.

https://reportaproblem.apple.com/?s=6

Many Games are free to download and play. However, these are often financed/monetised through embedded advertising. Free Games supported by advertising often have an in-App purchase to remove advertisements.

While in-App advertising should be age-appropriate to the game itself, this is not always well enforced. At best, advertising can be intrusive to gameplay.

If the App(s) in question do not have an in-App purchase to remove advertising, there is nothing that you can do to suppress the unwanted content. In such situations, your only option will be to delete the App - and find something more appropriate.

If your Gaming is using a web browser, you might try using one of the better Ad/content blocking Apps. One of the most respected within the Apple App Store - designed for Apple devices - is 1Blocker for Safari.

https://apps.apple.com/gb/app/1blocker-for-safari/id1365531024

1Blocker is highly configurable - and crucially does not rely upon an external proxy-service of dubious provenance. All processing by 1Blocker takes place on the device - and contrary to expectations, Safari will run faster and more efficiently.

Unwanted content is not simply filtered after download (a technique used by inferior products), but instead undesirable embedded content is blocked. The 1Blocker product creates a ruleset that is actually processed by Safari.

iOS/iPadOS has limited built-in protection from unwanted pop-ups and advertisements - described here:

Block pop-up ads and windows in Safari - Apple Support

The most likely cause is that your grandchildren are playing a free game that

is ad supported. Most such freemium games offer an in-game purchase to

do away with the ads.

maybe Block pop-up ads and windows in Safari - Apple Support