Not at all unreasonable for a child of any age.
I know some adults who believe similar things. Lots of them, actually.
Some elderly people I know even have a special building where they meet and pretend to talk to a baby they believe grew up, died, and then came back to life. They think they'll live forever if they just get into contact telepathically with this baby (who is still alive, although invisible) and do what it tells them inside their head.
Others, with a different building, meet there and separate men and women while they have a good wash, stretch out on the floor, and do the telepathic contact thing. (Though not with the same dead-and-alive baby, obvs.) Some of these people wear special clothes so other people know they believe what they do. (Well some of the women do, anyway; I'm not sure why they have different rules for men and women.)
Some of these people have special meetings in their special buildings at Christmas, some at other times of the year, where they sing special songs and generally have a really nice time. (And they have special books and other things, too.)
No harm done, as long as they don't go trying to kill people who don't believe the same as them. (Which obviously could never happen ... Oh, erh, wait ...)