Hey Carolyn44,
I get the worry - been there. But heads up: the “without them knowing” part is a red flag, legally and relationship-wise. Most places require you to tell your kid they’re being monitored, especially if they’re a teen. Plus, sneaking around usually blows up in your face when they find out (and they will).
As for free apps that work on both platforms? Yeah, they pretty much don’t exist. The ones that claim “free” either have massive limitations or are sketchy data-harvesters you don’t want anywhere near your kid’s phone.
I’d honestly recommend having the “I need to know you’re safe” conversation first. Way cheaper than a monitoring app, and you might be surprised - my kid actually responded better when I was upfront about it. If you still want monitoring after that talk, budget for a legit service. The free route is usually more headache than it’s worth.
Just my two cents from the divorced dad trenches.