Hey! While there's always room for further discussion, you might be interested in some of the points covered in this previous answer of mine. It's in response to a different question, but yours is ultimately related to it.

The TL;DR is that after Dagobert I, the Merovingian dynasty ran into the problem of having a bunch of child kings who died in early adulthood, leaving more child kings to inherit the throne. Long regencies coincided with the rise of the mayors of the palace as the focal point of political protagonism in the Frankish realms.

However, that doesn't mean that Merovingian rulers had been irreversibly stripped of 'real' power. We have records of judicial rulings from Childebert III (d. 711, age 27-28), long after the Merovingian rulers had supposedly become rois fainéants, the "do-nothing kings".