The child could use a ball or a stone to illustrate the point by showing that objects with mass have to rest on something.
وَٱطْلُبْ مِنْهُ أَنْ يُظْهِرَ كَمْ كَانَ صَعْبًا أَنْ يُصَدِّقَ أَحَدٌ آنَذَاكَ أَنَّ ٱلْأَرْضَ مُعَلَّقَةٌ عَلَى لَا شَيْءٍ .‏