The American Heart Association states,
"AHA Response: The AHA advises that pediatric AED pads should not be used on adults. The AHA 2020 Guidelines have a couple of recommendations that can help direct your options.
If neither a manual defibrillator nor an AED equipped with a pediatric attenuator is available, an AED without a dose attenuator may be used (COR 2b, LOEC-EO).
Use the largest paddles or self-adhering electrodes that will fit on the child’s chest while still maintaining good separation between the pads/paddles (COR 1, LOE C-EO)
When affixing self-adhering pads, either anterior-lateral placement or anterior-posterior placement may be reasonable. (COR 2b, LOE C-LD).
So, in summary, larger pads can be used on smaller people, but smaller pads should never be used on larger people."
So, if you have only adult pads, you may use them on a child. The pad placement on a child can be either like the adult placement (below the right collar bone/left rib cage) or like the child placement (mid-chest and mid-back).
You should check the specifications of your AED to confirm that the adult pads are useable for kids AND check that the dose (strength) of the shock is also safe for a child.