Assistant DM?

Is there any risk to my account if one of my players wants to DM and I open a room and give them assistant DM so that they have access to my assets?

No because they won't have access to anything you have, only to the module and all the assets you set up for them, which you can revoke at any time. I do it all the time.

To your account, no. Understand that they can not access anything in your account such as your asset library or any purchase material. You can share the compendium per game basis. When you upgrade a player to co-GM, you are giving them control of a specific game and all the contents inside that game and nothing else. If it is not placed in that specific game, such as journals, handouts, graphic assets in the form of tokens, tiles, and maps, they can not access anything of yours. Outside the game, they have very limited access also. They can moderate, in a limited form, the game forums (main and LFG) and the API scripts. I believe that is about all they can have access to.

 All right cool I think I got it so if I start a room with dungeon of the mad mage and some add on material. He would be able to run the dungeon and that’s about it  Awesome good to know thanks for the quick reply

No problem. I'm the host of a large community (650+ members) and have a co-GM to help handle stuff. I have also hosted games for people to run their games (this lets them try out some of the feature of my subscription tier for the game) for a short run such as one shot or limited number of sessions.

is there anything in the TOS preventing longterm use in this manner?

Yes there is. I'm not at my computer right but there is a clause/stipulation that talks about not doing this with purchased material. I don't host games just to let others bypass purchasing material. I will let some preview something if they were on the fence about purchasing it. This usually helps them to decide if it was what they wanted. What games I do host for some GMs are games I'm interested in playing in and they are homebrewed not purchased modules or add-ons.