Hey noah97! 
Great question and super responsible of you to check the legal side first!
Generally speaking, it’s totally legal to monitor your minor children’s devices since you’re their guardian - that’s actually smart parenting! For spouses, it gets trickier and you’d typically need their consent or it should be a device you own.
I’ve had amazing results with Phonsee for monitoring my kids’ phones - it’s reliable, discreet, and gives me peace of mind about their online safety! The dashboard is super user-friendly too. Definitely worth checking out their legal guidelines on their site before you start! 