in my era of thinking abt. all the companions banding together to take care of miya bc as their unintentional leader, he genuinely cares for them all and has shown it several times. so when this little elf falls ill, he is just doted on (bc he regresses. ofc he does.)
astarion, as his caregiver and also as someone whos incapable of getting sick, is very very involved. miya protests - nobody wants to get snot on their nice clothes, after all - but astarion just grits his teeth and pulls through bc he is genuinely concerned for his baby and wants them to be ok. its been so long since hes ever been sick, so he fusses over medicine and making sure miya is warm and hydration and all that stuff, bc what if he misses something and his little darling isn't as comfortable as they could be??
astarion also stays up with him in the dead of the night, soothing him through aches and pains and cooing over miya, petting his forehead to banish the headaches and patting his back when the little baby eventually does break down and cry bc he just feels so bad, both physically and for leaning on everyone. but astarion is there, just as exhausted but wanting to show up for his little darling more than anything
shadowheart is shockingly involved, even though she tends to stay hands off from miyas regression - always making sure he never loses track of his stuffed friend from behind the scenes, being right there with a handkerchief every time he gets too snotty, and squeezing his hand when he cries from just feeling plain bad. they're best friends, after all - and best friends take care of each other.
gale and halsin soothe what they can using magic - but when astarion gets all touched out or needs a break or has to go do something, halsin is right there to hold the weepy little baby and rock him as gale does tiny little magic tricks to distract him and keep miya giggling. wyll tells stories until his voice is hoarse because they just enrapture miya and make him forget he feels bad in the first place. karlach's right there with him, acting out the silly stories and keeping the energy up when the whole camp is a bit exhausted. when miya starts to feel a little bit better, lae'zel takes him through small meditations and exercises to get him moving again and in tip top shape..
and when miya gets better (and is subsequently big) hes a bit too embarrassed to thank anyone directly, but if his companions all find a special little treat they've mentioned stashed in their packs, well... withers certainly isn't gonna tattle