The difference is not in grammar but in meaning.
"I told you" means "in the past I verbally passed some information to you".
"I have told you" is emphasizing the fact that I passed some information to you before. It usually means the speaker is angry or annoyed about having to tell the other person again.
However, normally when someone gives someone a warning about something, and the person doesn't listen and what they were warned about happened anyway, the first person says "I told you so" or "I told you that would happen". They wouldn't usually say "I have told you".
For example:
Mom wants to go to The Grand Restaurant for her birthday. She told her son 2 weeks before her birthday. But the day before her birthday...
Son: Mom, did you tell me where you want to go for your birthday dinner?
Mom: I have told you!
Son: When did you tell me?
Mom: I told you 2 weeks ago when I picked you up from baseball practice. How did you forget?
Son: Uhh...
Note you can't say "I have told you 2 weeks ago." You can't use the present perfect with a specific time point.
In the given example with the child falling, using "I have told you" would make more sense in this situation.
Child: Ouch! I'm hurt! Why didn't you tell me to be careful?
Parent: I have told you/I did tell you! You didn't listen to me.
I would actually prefer to use "I did tell you" in this instance, but "I have told you" has the same meaning and purpose: emphasizing that the speaker already said something to the listener.
I hope this is helpful.
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