Crossbreeds cannot halter unless they are in a campagin and purebreds need to be registered in their breed registery to halter show. 

The wiki can tell you more 

So with Smell the Rug, I had to enter a comp through the halter show tab first. Is there anything I can do with Pixie Pie or can she sit till she's two? I just wanna make sure I'm doing everything I can for/with her. Thank you once again!

As Maki had said, Crossbreeds need to be registered into a Campaign, otherwise they cannot do anything until 2 years of age. I looked at her more myself and it seems the foal in question is part Warlander along with some Tennessee Walker and Irish Cob. I do know there is some horses with IC/TWH in them as part of Campaigns but only one is part Warlander and that is Warbreds; the foal needs to be 50% Thoroughbred, 25% Friesian and 25% Andalusian, since she is none of those things, she cannot be registered into a Crossbreed Campaign.

Crossbreed Campaigns really have strict %'s the horses can enter, some are simple half and half, others can get more complicated depending on the breed in question, even more complicated than Warbreds, so in short, there is nothing you can do with her until she is 2 years of age.