By leveling Myrina up to 6 stars? I only have a +0 Myrina and I don't understand the mechanism about which child the system will choose to use drive skill.
Ai is quite random and any child can drive at any specific time when is allowed (100% drive charged) Auto Drive and Auto Skill has to be OFF then you may plan their strategy attacks weather to tap attack or slide to heal and which child drive to tap in order to boost drive to fever.If you feel the child's are strong enough (level 60 class S with uncapped +6) you can leave them on auto levelling gameplay and the AI does it for you.I do use auto skill / auto drive but slow down the game speed to x1 instead of x2 / x3 in this case I can still view my battle and tap or slide when I feel its necessary and I can time my slide attack properly in order to reach fever.
I just change x3 speed to x1, have the support (healer/buffer and debuffer) on the outsides and drive myself. There's been a plethora of times when half my team outchere on 1hp and elysion uses her drive 😭
I like your idea placing child near at the outsides left right. Many times my Chang'e auto drive but when I accidentally tap it to stop and is only a bad percentage drives and all my child doom because can't reach fever in time 😭
For me the A.I uses the highest attack character for the drive, then on the second drive it chooses the second with the highest attack. Remember that when you dirve with a child he/she will have a downtime before you can drive with him/her again even if you are at 100% drive gauge, so the A.I will just go to the second highest attack character on the team. At least that is how it works for me.