Add check box for "language delay"

Nobody has claimed this yet.

# TL;DR
Adding a check box for parents to use if their child is "language delayed".

# Narrative
As a researcher who studies primarily language delayed toddlers, this would allow me to target this population for data collection.

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

- [ ] Tests for new feature
- [ ] QA on new feature
- [ ] Sign off from Kim

# Implementation Notes
Leave important notes/suggestions here for implementation (important files, suggested approaches, etc.)

The issue names no files, tests, or entry points. Start by locating the existing child or participant data fields and comparable checkbox features in the API, then determine how this value is exposed for research data collection. Done means the checkbox is available to parents, covered by tests, and has completed QA and Kim's sign-off.

Written by the indexing model from the issue text.