Advice re. teaching truth to toddler

[Atenco]

My dd and dgd live with me. Dgd is two years and three months old and is a lovely, happy child. But she has lately started "exaggerating" a lot about supposedly being hit. For example yesterday I gave her a very light playful tap on the nappy and she starting going "Granny hit me, aw aw" and burst into tears. I'll know not to do that again.

My dd thinks that maybe she or the father over-reacted the first time she told them she had been hit at nursery and this is why she is doing this.

But our concern is when something happens that we do not witness, if she is going to exaggerate everything so much, how will we know when it has indeed been a serious incident. How do you teach such a small child not to cry wolf?