For example, grapefruit essential oil could increase the effectiveness of some prescription medicines just like eating a grapefruit would.
Also not all oils are safe for ingestion. For dōTERRA oils, if there is a supplement fact on the bottle then you can take the essential oil internally. If there is not it will say for external use only, these oils should not be ingested.
For small children I would dilute any essential oil you want to use with (a carrier oil) fractionated coconut oil, grape seed oil, almond oil, extra virgin olive oil (even though it's thick) before applying to the child.
Also, if you get an essential oil in your eye or it feels too strong on your skin, (too warm or too cold) you can add a carrier oil. I prefer fractionated coconut oil for the eye, but any of them will do for any place else.
Be healthy, be safe.