Yes definitely, if you say to a child that come here I will give you a toffee if you not give him toffee then you mislead him. In Islam there is no permission of lie even in Jokes also...
What is Lying?
Lying is the complete opposite of truth. So, anything that is untrue and deliberately intended to mislead another person is a lie. A lie, therefore, can be anything spoken or written that is totally or partially baseless, unreal, made-up, distorted or exaggerated; for example, if someone were to deliberately state that a five foot pole was a ten foot pole, then this would be a lie. Similarly praising someone out of proportion is a form of a lie.
Lying is not limited to the serious matters only but people also lie and make them as jokes. Even this is not permissible in Islam because even in joke, we can hurt someone or might be our own selves.
It was narrated that Ibn ‘Umar said:
“The Prophet (peace and blessings of Allah be upon him) said, ‘I joke, but I speak nothing but the truth.’” (Narrated by al-Tabarani in al-Mu’jam al-Kabeer, 12/391)
This hadeeth was classed as hasan by al-Haythami in Majma’ al-Zawaa’id, 8/89, and classed as saheeh by Shaykh al-Albaani (may Allah have mercy on him) in Saheeh al-Jami, 2494)
The Prophet (peace and blessings of Allaah be upon him) said:
“Woe to the one who tells lies to make people laugh, woe to him.” (Narrated by Abu Dawood).
The Prophet (peace and blessings of Allaah be upon him) said, warning against this kind of behaviour which some jokers are accustomed to:
“A man may say something to make his companions laugh, and he will fall into Hell as far as the Pleiades because of it.” (Narrated by Ahmad).
Source: Islam Questions & Answers