Open the general settings and click Restrictions. You'll have to create a special 4-digit pass code; pick one the kids can't guess. From there you can disable access to Safari, YouTube, iTunes, location notification and the ability to install apps.
In addition, you can disable in-app purchases and limit TV shows, movies, podcasts and apps by rating or explicit content warning.
It's not perfect and won't deter determined kids for long, but setting this up will offer some peace of mind.