Ancient Egyptian Hieroglyph by Kathy Allen is probably the best children's book I have ever seen that addresses Egyptian Hieroglyphs. This book is geared toward 3-4th grade or a little older depending on the skill of the child. This book talks about the history and uses of hieroglyphs. Not only is the ancient history covered but also the translation of the Rosetta Stone. Another interesting historical feature of this book that I haven't seen in other books is that it talks about Coptic writing and its link to hieroglyphs.
What is most impressive though is that this book has broken down translation and reading basics into a clear and comprehensible series of steps. This book makes hieroglyphs more accessible due to the simplicity and completeness of its coverage.
I definitely think this is a book for any child interested in Ancient Egypt. It would also be a great starting place for an adult to get the basics before moving on to the more complicated adult translation books.