My neighbors found a tiny rodent in their garage and my kids are looking after it. They looked up on the internet that the best would be a surrogate mother (don't know of any); wildlife rehabilitators can be good but often feed rodents to owls and cats (I thought this would be a noble thing to do, but my kids were unpersuaded).
Anyway, I wondered if it were actually just a small adult rodent of some sort rather than a tiny baby (as it appears). In the case of the former, it would be better to release it immediately.
The location is Orange County, California. Attached is a photo: