What is wrong with giving juice in a bottle or propping up a bottle of formula for the child to drink at bedtime?

I believe the answer is: Sugar bathes the teeth, bacteria grow, acids are produced from sugar, and acids dissolve tooth enamel.

This would make the composition of the teeth become generally weaker and would make it easier for children teeth to decay. In order to prevent this, the parents have to teach the children to no longer eat anything after brushing their teeth.