If you are into the $100+ range for considerations... I'd just spend ~$150 and get them all iPod Touch, 6th Gens.
It has two functions that will be helpful:
1) "Find My Phone" - which is GPS tracking
2) Family Sharing
You can set them all up as if they are your "kids" - which they kind of are, right!.
The iPod "GPS" is going to work like any other device... as long as battery is good & function turned on, it'll work.
We bought an iPod Touch for our 9-year old daughter (great option in place of a "phone") and she takes it everywhere.
https://itstillworks.com/locate-lost...-gps-1885.html
With the family sharing, you can link a bunch of them... and also share apps - which can be nice.
Also - of course this will be a great keepsake - with lots of future use.
I once researched small-device GPS for when my kids were little and I was paranoid.
I tried finding if companies made shoes where the GPS could be hidden - but the big issue is the "battery" which takes up size.
Chargers for an iPod (Touch) will also be stock-available... even in Mexico City - something you'd want to consider.
Better to think about a "major manufacturer" device,
so that you can get support FAST if you need it.
[Also - my cousin-in-law is a BIGGIE at apple, so if you DID lose somebody, I could help you get a little more serious help in GPS tracking.]
BTW - if you happen to be strongly considering this...
I have a 20% off coupon for Target (good ONLY thru Dec. 8th) - and you could consider ordering a bunch online from there.
Those are running $185 currently.
You could do the math on 20% off.
https://www.target.com/p/apple-174-i...n/-/A-39595410
The hitch is... I'm gonna make you drive to Scarborough to pick up a case of Panini hockey stickers from a shady location.
(Seriously - anybody near Scarborough???)