'Ubada bin As-Samit may Allah be pleased with him narrated: Allah's Messenger (PBUH) said,
"There is no Salãt (prayer) for him who does not recite Ummul-Qur'an (Surat Al-Faatiha)".
The above mentioned Hadeeth is a clear proof that without reciting Surat Al-Faatiha, prayer Is not valid. The companions of the noble Prophet (PBUH) and the followers of the Companions and most of the scholars believed in this practice. The noble Prophet (PBUH) ordered to pray with the recitation of Al-Faatiha, in every Rak'at.
On whether one recites Al-Faatiha when following an Imam, Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: The (scholarly) opinions boil down to three: two opposite views and one in the middle. One of the two opposite views is the view that he should not recite behind the imam in any circumstances. The second view is that he should recite behind the imam in all circumstances. The third view, which is the view of the majority of the early generations, is that if he can hear the recitation of the imam, he should listen attentively and not recite. Majmoo‘ al-Fataawa, 23/265
And Allah knows best.
Although a pre-pubuscent child is not obliged to do Wudhu nor offer salah ,it is commendable to encourage them to do Wudhu & perform Salah.