If I use a djinn or a psynergy move that increases my attack, does it also increase attacks of summoned creatures and djinni? Or is it just my party's weapon attacks?
A Djinn's (and their summons') attack strength is increased by the Elemental Power of that element (e.g. Venus Power) or elements, since they do elemental damage.
You get +5 Power per Djinni set, you can gain more Elemental Power by summoning or equipping certain items that increase elemental power.
To learn more about Elemental Power, consult your local Golden Sun Wiki