Mike: Okay, as much as I am a sucker for tiny!Anders calling Mike "Mikey" in the smattering of deaged!Anders fics on AO3, I had a thought the other night. From what we see in the show, by the time Anders would start talking, there isn't anyone in their lives who would likely call Mike anything but Mikkel. Both their parents solely use Mike's full name, and they're so young that they probably aren't interacting with anyone who'd call Mike something different than what their parents use. (Even Olaf calls Mike Mikkel a decent amount during the show, so if/when he'd visit during this period, it's unlikely he'd be the odd one out here.)

If Anders only knows Mike as Mikkel at this point, I doubt he'd be able to say Mike's full name at first, since Mikkel is a bit complicated to pronounce for a kid just learning how to speak. So… proposal that tiny!Anders' attempts to say Mikkel came out more as "Mik", and that's what stuck. Only gets used for a couple of years, and Ty/Axl never pick it up, since Mike has already started to go by Mike at that point. Gets used on very, very few occasions post-show once everyone's on better terms for the same reason as Andy below, with the additional caveat that Anders opts not to use it if it's not just him and Mike talking.

Anders: Two different degrees of nicknames here. Name is slightly shortened to "Ands" on occasion, mostly by Mike and Ty. Stemmed from their teenage years, stopped being used when Anders was estranged from the family, and slowly worked its way back in over time. Becomes more common post-show when everyone's on better terms again. However, Anders' original nickname was "Andy", started by Mike when Anders was very young. Lasted for a good chunk of their younger years + was also picked up by Ty when he started talking; stopped being used sometime mid-late elementary school. Not used at all for multiple decades; used on very, very rare occasions once everyone is on better terms — means either there's a rare emotionally open moment or shit has hit the fan horribly.

(PS: Dawn is also allowed to use both nicknames. She tries not to abuse said power so Anders won't get defensive about it, but Anders secretly likes it.)

Ty: Solely goes by his nickname; doesn't like being called Tyrone. Put up with their mom using it, but that's the extent of it. None of his siblings have ever called him anything but Ty; even when the brothers are arguing, using Ty's full name is off limits. (Mike did once, very early on into being Ty's guardian. The situation was immediately made ten times worse, and that was never repeated again.) Dawn also only calls him Ty.