An RTG would not create a cherenkov glow. An RTG contains the radioactive material in question and uses it to generate electrical charge, which is then used to power conventional devices - such as, say, an LED.
I don’t believe anything as small and light as the atomic nightlight or lamp would create a cherenkov glow, and if it did, it almost certainly wouldn’t be safe. Cherenkov radiation is formed when matter, which is usually a charged particle and most likely an electron, is forced to go faster than the speed of light in a given medium. They would not contain 238Pu, because 238Pu is an alpha emitter, and alpha particles are just waaay the heck too slow to create Cherenkov radiation. (Or to put it another way, if you are seeing Cherenkov radiation coming off 238Pu, you and everything you know is turbofucked.)
What you want is a beta emitter with a long half-life, as beta particles are fast-moving strong electrons, and probably some kind of bespoke metamaterial medium designed specifically to have a low speed-of-light whilst still being transparent. This would be very dangerous, because beta particles are one of the ones that can penetrate skin and cause spontaneous mutation of DNA strands… But it’s also able to be stopped by thin shields; the question is whether enough enough of it can be stopped…
So, to recap, to have an “Atomic Lamp/Nightlight” as currently described and modeled in-game, you require:
A radioactive isotope with a usefully long half-life (which will not be on the order of millions of years, but a couple of decades should be doable) that emits beta particles and is not in and of itself particularly horrific, nor does it decay into anything particularly horrific. Remember, the longer the half-life, the weaker the radioactive emissions, and this is a fundamental law of nuclear physics. Strontium-90 is a good candidate, as is Caesium-137 with a half-life of ~30 years each and as long as they are safely contained, perfectly safe. When is not properly contained; very dangerous, such as in the Goiânia Accident.
You would also need a bespoke metamaterial medium and shield, designed such that the medium has a very low transitional speed-of-light through it whilst still being transparent or translucent enough to meaningfully see the cherenkov glow, and a shield which is transparent but is a good enough beta-catcher to allow the device to be nominally safe.
I do not envision the Nuclear Regulatory Commission or the European Commission approving any such device for home use, but then, I would also never envision them permitting 238Pu-powered automobiles. Clearly, the Rivtec Corp has achieved megacorporate status sufficiently to flagrantly dictate legislation, so, that is what it is.
Even so, it would be far, far simpler to just call them an RTG which operates upon conventional principles; a pellet of some nice, hot-decaying material - which could still be 90Sr, but other likely candidates are 238Pu, or even 241Americumium - surrounded by thermocouples which turn the decay heat into electrical energy, hard-shielded by something conventional like lead, which then powers an ordinary device such as an LED or hotplate.
