Hey, I am not a parent so I wouldn't have much experience with this but here is what I do know.
Have you looked into the hardware you plan to use?
Have you looked into the companies behind said hardware?
Have you looked into what may happen to the GPS data?
You might not care about this and that's ok but I do think you should look into how safe the hardware you plan to use is. That said no child I know of would willingly wear a tracker without explanation, explain your motive behind it to the child. Do not under any circumstance lie about what it is