We recently started using Angel Sense. There is a monthly fee, but they run specials all the time so you might catch a deal. It looks like a little cell phone in a rubber case that cannot be opened. You put that into a cloth belt and attach it on them with a magnet thingy that they (nor anyone else) can remove without a special device! The app goes on your smart phone and it shows you on a map (like Google Maps) EXACTLY where they are. Ours has a feature where you can call it (it actually rings) and you can talk to your child, or you can call it (where it does NOT ring) and listen in with nobody knowing. Our son is still getting used to it, but so far we like it. We recently went to an amusement park and my husband took him to a bathroom that was right at an exit gate and I got a text saying he'd left the park (he hadn't but the tracker noted the gate as an exit). It doesn't beat eyes and ears at all times, but when you need it, you'll be glad you have it.
Best to you.