There are two types of crow's feet lines on the sides of the eyes. Those that arise from expression muscle movements (dynamic wrinkles) and those that are present at rest (static wrinkles). Botox and other neuromodulators, such as Dysport and Xeomin, work only on softening overactive muscular movements and hence only can be expected to improve dynamic (movement-related) wrinkles. For static wrinkles (the creases and etching-like marks that arise from years of "folding" our skin while smiling and emoting), we need additional approaches. In general, fillers are used to plump up the creases.
I have found that performing a Nonsurgical 3D Vectoring Lift in this region first (before using fillers) often provides the necessary projection to soften the wrinkles--which may then be treated with fillers (if still even necessary) to further obliterate them.
Analogous to the approach used for the Nonsurgical 3D Vectoring Browlift, tiny "threads" or "strains" of a volumizing filler are placed strategically across the crow's feet region, which yield immediate results and also give rise to longer-lasting results via stimulating subsequent neocollagenesis (new native collagen formation) that can lead to further tissue retraction and lifting of the target areas.
Consultation with a board certified aesthetic physician experienced in all these techniques is strongly advised to ensure that the best approach or combination of treatments be tailored to your individual needs.