There is kind of one problem though, since as you mentioned it is still a dnd campaign, if your friends choose their characters to be warlocks or orcs, they can easily beat it all through with teamwork.
My suggestion could be that you can make some rules for the party about how they choose characters and such, because I am darn sure if anyone chooses warlock(capable of magic) or orc(super strong with rage attack) they might be able to instantly kill somewhat everything on level one.