A Good Translation
My suggestion for the best Bible for children to read is, surprisingly, the King James translation. My recommendation is based on two primary reasons.
For the sake of children, the KJV Bible is one of the best translations as, although it has a few errors, they are minor and well known.
Additionally, the beautiful, poetic language of the King James Version has proven easy to memorize even with the strange use of all the verbs ending in "eth" and "est." The KJV can sometimes be more easily retained and remembered, even more so than other, more easily understood Bibles like the New King James or New International Version.
Teaching Tool
It should be noted that it is remarkable that many of the best scholars of the last century, and even before, learned to read using the King James Version. Countless "average" humans as well learned about the English language through reading the Bible, as children, in the home and at school.
To be fair, the only major difficulty with the KJV Bible is that it uses archaic words and grammar whose meaning has changed over the years. These words and their modern meanings, however, are also known.
Understanding Scripture has never been easier, with tools existing that were not available 100 years ago. For example, a relatively inexpensive Parallel Bible translation can help in comprehending Scripture.
The text of the KJV Bible and another translation, such as the Amplified version, are placed line for line in parallel columns on the same page. This type of volume allows one to read a passage in the King James Version and then to see how the more easily understood translations have stated it.
Newer Translations
With the KJV as a baseline, one can see the flaws in the new translations that are touted as the best, yet better understand the KJV. Zondervan publishes a New International Version, New Living Translation, King James Version and New American Standard Version Parallel. They also produce a New International Version with a KJV Parallel as well. There are probably many other publishers who have similar offerings.
For any student of the word of God, it is recommended that they purchase a Strong's Exhaustive Concordance of the Bible. It has every word in the King James Version numbered and related to the original Hebrew and Greek accompanied by dictionaries giving the meaning and pronunciation of the transliterated words. One can then see how a Greek or Hebrew word is translated in many different contexts.
Computer Tools
For computer literate children, there are many Bible study programs available which offers many different translations, Strong's Concordance and many other reference materials. This type of tool has the added benefit of speed in regard to searching and scanning Biblical text. Easy-to-use and free study programs also exist such as E-Sword.