How to store historical data using Django
Forget about Django's ORM (which is really just a very thin wrapper over your SQL database) and think in terms of relational database. Here you obviously have two entities: "kid" and "height". You also have the following relationships: a "kid" has zero, one or many "height", a "height" belongs to one single "kid". This indeed translates to a "kid" table  kid(+id, name, dob, ...) and a "height" table with a foreign key on kid: height(+id, kid_id, date, value).
How to write this using Django's orm is quite straighforward so I'll leave it to you.
I am new to Django and am trying to figure out how to store historical data and be able to reference any data point from a date in the past. Say I want to store data on a kids height change as he grows and be able to compare his height at day 400 to his height at day 6xx. How would I set this up in Django? Would there need to be a foreign key relationship between height and the kid?