If the mural is thin, then Mod Podge might be a good fit. It is often used for photos. You can get gloss or matte, you can get interior or exterior moisture resistance. All non-toxic. Look online. It does tend to take on the texture of the surface it is on. Paper on wood, when mod podged, will be a bit textured like the wood.

I sealed some wood hide boxes with mod podge. It works quite well. I used it because of its non-toxic properties. It dries rather quickly. In 24 hrs there is no smell. It can be sanded and recoated anytime. Works over water-based paint too. Or under it.