If someone accept Jesus as Lord and Saviour based on NDE, I wouldn't keep thinking about their experience.. Wouldn't spend much time thinking whether they really saw Jesus or there is some other scientific explanation.. I would be more interested (or rather burdened) about their walk with Christ.. The process of sanctification is the assurance of their justification.. That would hold the same for that person as well.. The important thing to consider is, are they progressing in their walk with Christ..
Regarding the book about that young kid, I would prefer to stay away from it.. My personal opinion.. There is no way to have extra biblical revelation.. Everything is revealed in the Word of God.. No one can add to that.. Absolutely no one.. We have to be like Bereans.. Test everything against the Word of God.. Many false prophets will come.. In the name of Jesus.. How do we identify them? Only Holy Spirit can gift us with that discernment.. And I strongly believe Holy Spirit illuminates the Word of God to us so that we don't get deceived by false teachings..
So it does not matter if NDE testimonies are from mega church pastor or someone as innocent as a 4 year old.. If it is unbiblical, stay away from it!