It's great for kids and adults to create their own affirmations.
Steven Covey says a good affirmation has five basic ingredients -
it's present tense
'It is deeply satisfying (emotional) that I (personal) respond (present tense) with wisdom, love, fairness, and self control (positive) when my children aren't behaving appropriately'
He also talks about how we can visualise this. So think of a situation you'd like to improve or a goal you'd like to achieve perhaps.
Visualise yourself in the situation - how do you look, what do you see, touch, smell, feel, what qualities to you show, what script can you write to go with this visual experience, is it inline with your personal mission and vision statement (see previous post).