It's sometimes inevitable to keep your little ones away from your phones. If they enjoy watching singalong or story-reading apps, they can't help themselves but to press the little round button. And once they are in your home screen, well, you know what's next.
iOS holds an accessibility option named Guided Access, which allows the parent/guardian to limit what can be interacted within the app itself. You can mark the areas to allow taps, or just block interactions altogether – This works for me best because it means my daughter won't fiddle around with any other apps or accidentally call anyone.
Guided Access can be enabled from Settings > General > Accessibility > Guided Access, with nice and clear instructions on how to set it up.
*Although Guided Access is a handy little feature, I highly discourage parents from letting their toddler stare at a phone screen for extended periods of time. It's unhealthy for them and it won't do any good for your relationship.