A fascinating notebook entry! Thanks for sharing this tidbit.

> A theory about how Spanish children to learn.

This is a very general statement about all Spanish children throughout all of time, so you'd use the present continuous learn here. It's also a title, so there would be no period at the end. Also, the first and last word and all internal words (except articles and prepositions of fewer than five letters) would be capitalized: A Theory About How Spanish Children Learn

You could also make the statement more general by saying Spanish-speaking children, since speakers of Latin American Spanish children doubtless experience the same phenomenon.

> When Spanish children have between three and five years old more or less,

More or less is a nice phrase to use here. Do set it off with a comma before as well as after, though. Also, the be-verb is used in English for age: When Spanish children are between three and five years old, more or less, they . . .

> so they couldn't learned it by listening.

When using the past participle (learned) like this, you'd need the auxiliary (helping verb) has in the appropriate person (como he, hemos, ha, han, has, habéis): so they couldn't have learned it by listening.

A theory about how Spanish children to  are  learning.

When Spanish children have between three and five years old  more or less, they make an interesting mistake about the participle form. There's an irregular participle "roto" broken, for example, they say "el juguete esta rompido", of course that's wrong, the right one is "el juguete esta roto", but 'roto' is an irregular participle in Spanish. It's really weird because no one else says 'rompido', so they couldn't learned it by listening. According to it, some experts comment that maybe Spanish children  learned first grammar structures first and after then the vocabulary maybe!