can we have "test baby" accounts to try things out?

# TL;DR
for testing out how an experiment feels on the participant side, having a "test kid" would be great

# Narrative
As experimenters, sometimes we want to run through a study like the participants would, and i think right now we just have to create a 'fake child' of whatever relevant age, but then that child will get emails asking them to participate in studies, etc. If instead there was some way that researcher accounts could have a 'test baby of age x' kid, this could be avoided

# Acceptance Criteria
A precise description of how/when this issue will be considered closed, e.g.:

- [ X ] Tests for new feature

# Implementation Notes
no strong feelings on implementation

The issue does not identify files, tests, or an entry point. First clarify how researcher accounts create and use test children, including age handling and preventing participant emails. Define concrete acceptance criteria and add tests for the agreed behavior.

Written by the indexing model from the issue text.