brushing teeth

I’m trying to teach Crystal to brush teeth, by giving her a yellow toothbrush.
The brush is a little bit big for her tiny mouth though.
Her brother and she has a favorite Chinese song,
“wash…brush…brush…” (洗刷刷…洗刷刷…)
I will sing it for her and show her while somebody is brushing teeth.
She would watch attentively and put the toothbrush in her mouth and bites it.
Her grandpa doesn’t agree on my teaching her to brush teeth.
He said, it’s unsafe in case the brush hit her eyes.
I say nothing, I don't want to argue with him, but I will still teach her,
as long as I watch her doing it.
I saw her brother 4 upper-front-teeth, it’s totally covered by un-brushed food,
and (to be honest) it looked so horrible.
When she is used to and understand this activity,
I will try to brush her teeth.