You may need to use manual focus if the bubbles are your focus target. Just a guess as I have not tried this, but there is not much there to focus on.
Instead, I would place the child in profile such that the bubbles were blown across the frame so that they are in the same plane as the child, and focus on the child. As you move away from profiles, a smaller aperture will get you a bigger depth of field to include the child and the bubbles, assuming you want the child in focus.
---------+---------+---------+---------+ Joseph K Seattle, WA, USA