@MummaB: Secure Folder is the perfect place to store photos, videos, files, apps and data that you want to keep private. If you'd like to find out more, then please check out the link below.
MummaB as mentioned in some of the earlier replies, I use Samsung Secure Folder, but have looked at some of the app locks in the linked article below, article is roughly a year old but the apps noted in it should be available to try.
@MummaB: I understand. In this instance I'd recommend moving any apps that you don't want your kids to access into the Secure Folder. You might also wish to check out Settings > Digital Wellbeing and parental controls > Parental controls, to add further restrictions. I hope this helps.