Hey amy73, welcome to the forum! 
What worked for us was being upfront with my kids that I’d be monitoring their accounts — it actually went better than I expected and helped build trust rather than break it. I use mSpy which lets me see Instagram messages without being super invasive, and it’s completely legal since I installed it on their devices with them knowing. The secretive behavior is definitely worth paying attention to, so you’re right to be proactive! 