Aside from putting her in Dalish armor, which they did?
True, but a bright green facial tattoo is going to grab attention where armor might not. Who knows what they were thinking, or again, if they were even mindful of any possible inconsistency at all when designing Dalish the Charger.
She could have done something that marked her as more 'deserving' of being kicked out instead of the child mage. Some sort of wrongdoing- perhaps she broke a rule or something.
So there is a 3 mage rule. Assuming that includes the Keeper themself, that would consist of: the Keeper, the First, and presumably a spare in case something happens to the Keeper's first.
The Arlathvhens happen once every 10 years, right? What if the clans can't afford to wait for the next one to swap mages? That could account for this 3 mage limit.
Dalish could have been the spare, and the new mage child could have replaced her as the new spare. If yet another mage child is born (mage child "B"), perhaps that child could replace the mage child "A" who replaced Dalish (who could by then be old enough to have his/her own Vallaslin) and is then kicked out just as Dalish was. Mage child "A" would be old enough to have a better chance of surviving than mage child B.
Or perhaps by then, an Arlathvhen would have occurred, and mage child A could be transferred to another clan.
Or, if mage child A is still too young to fend for themself, they could choose to get rid of mage child B, since mage child B would be a larger drain on resources since they are younger and more time is needed before they become a productive adult member of the clan. That could have been what happened to Minaeve.