I think everyone is different and no one is wrong with what they think is right for their kids as each child and family is different.

I wouldn't really bring the topic of conception up (no matter how my child was conceived) unless my child asked me some questions about it.

If they were under the age of 6 I think I would be saying that mummy and daddy wanted you somuch and loved you so much that they had to get the doctor to help them put you in mummy's tummy or something like that. I think that it may confuse them to go into too much detail about IVF, etc

If your child is curious then they will ask more questions and there is nothing stopping you from answering them.