I don't chase people around to heal them. keeping track of 7 children can be more trouble then its worth. In the case of adds I focus on the tank(player) since the co-healer can babysit the other 5 since they're not supposed to take damage past basic raid aoe. In the event the other healer wants the tank I've no qualms doing the other.

As a healer that tries to generally position in a central spot to reach everyone if you're outside of my range its your fault. On spread out adds I just do what I can, but peeps gotta remember there's no heal that's the size of the entire arena so be mindful.