Hi Trishon, well that depends a lot on what the child knows already about the Bible and spiritual things in general. I think it would be important to stress that the Holy Spirit is not only a force, a power but actually a devine person, as much as God the Father, and God the Son. One could also explain that the Holy Spirit has a special relationship to the believers: When someone believes in the Lord Jesus, the Holy Spirit lives in that person, guides, helps, protects, intercedes, etc. The prime goal of the Holy Spirit is, that we get to know the Lord Jesus closer and that we live to the glory of God. Hope that helps!