Kids : a hebephile's primer...

First, unless she's a freaking retard, the average three year old has stopped "toddling," and started "walking."

Anyways...

If anyone here has an explanation for such behavior

Well, for starters... people tend to be a bit more batshit insane when they're cut off from human contact than when they're not... which means that the absence of moderate neglect results in a person very different than its presence. Even in short-term scenarios. People recover fast in the right environment.

Moving back on to the topic of your story, however...

she came back and told me that the 3 year old had spent all of that time opening and closing the bathroom door non-stop. She was doing this for well over 15 minutes, and I fail to see the point or purpose of such actions.

Lemme guess - it had no latch and swung freely?

You realize that most toys and games work on the same action? Bouncing a ball against a wall.... push.... catch. Those little ball 'n paddle things... hit... catch. Tetherball... push... catch. Yo-yos.... push... catch.

You may not see the point or purpouse - but you've got a lot more people of all ages to talk to about it if you want to discuss the nonsensicality of the mechanics of the game...

Later, Terra came back to where we were sitting and my friend smiled and said "hi" to her. Terra simply shook her head in response. Yes, I was surprised by this reaction too...why would someone, even a toddler, shake their head as if to say "no" in response to someone saying "hi" to them?

"No" is...

1. A binary response.

2. A command.

Either way, it works... as a response...

"Salutations! Initiation of discourse at this time?"

"No thank you."

...as a command...

"Hello..."

"No!"

If you think that's nonsensical, please tell me how it's sensical for a parent (you know, the person she's learning the language from) to shout "no" at them when they play with the stove/breakable objects/etc, when no one even asked the parent a question about anything.

(and thirdly... "are you high?" "nope." Buuut... we'll just assume that's not it)

Anyways... I kinda like her sister. Her sister sounds pretty cool. I like her sister's attitude towards young children. It's a good one.