In this example, it would certainly be more moral to honestly convince the child than to lie in order to achieve this goal.
Here it is interesting to consider two elements of Kantian ethics.
- The Universality Principle - if this action were universalised, would this be acceptable?
If applied to this situation, lying is no doubt not the most morally supported solution.
- Personhood & Means to an End - Kant argues that it is immoral to use someone simply as a means to an end
i.e. Using the child as a means to an end would not be considered moral in this situation
In conclusion, truthfulness, simply as a consequence of these principles, would be considered a more moral solution than to lie to the child.
A different approach would be from the perspective of utilitarianism: which action will result in more morally acceptable consequences in the future?
In this case it is less obvious but nevertheless quite likely that truthfulness will help the child down the line (note that this is by no means guaranteed, but still a fairly reasonable approach).
The importance of truth in ethics arises simply due to the fact that alternatives to it either are not considered equally morally acceptable or can be expected to have more detrimental consequences int the future.