Emotions are by nature very personal, and can be hard to understand, control and give to God. Christina Fox's book is designed to help children open up to their loving Father with their deepest hopes and fears. He is our closest friend, and that is often tricky for children to fully trust when they neither see nor hear Him.
I'm sure many children will find it hugely helpful, but in our house we found it a little older than the 4-8 suggested age range. It is probably best for children who can read confidently by themselves. Moreover, it was clearly written for an American audience (understandable since the author herself is American), which can be somewhat jarring when read here in the UK.
There are many excellent Christian kids' books out there, including about emotions. They can be an incredible help for children in getting to know God, so I would recommend you try as many as possible. This may be the one that really suits your children.
This book is available to purchase from Christian Focus.
Miriam Montgomery, Free Church Books