Student or Learner
Start of a conversation: "When you were a child, your parents would have told you not to play in dirt. Isn't it?"
Shouldn't it be "will" in the place of "would"? Please add some explanation also.
Your parents will have told you not to play in the dirt when you were a child is possible.
Here we have will have told expressing the present certainty of a past situation. it is not future perfect in that context. Would have told shows less or past certainty.
If you begin the sentence with When you were a child, you are placing the whole situation in the past, and would have told shows certainty.
As the above post says, using will does not make a future perfect as we're referring to the past.
Both will and would are possible auxiliaries but of course the best choice depends on exactly what is meant.
Without further context, (this doesn't sound like the start of a conversation!) I suspect that will is the desired auxiliary but I think it would be clearer to rearrange the sentence a little by stating the certainty of the verb before the past time reference:
Your parents will have told you when you were a child not to play in the dirt.
This phrasing now gives us, in order, certainty (will have told you), timeframe (when you were a child) and then the main message (not to play in the dirt) at the end.