Is it a condition to accompany the funeral procession from its home to the mosque to obtain the first Qirat of reward? Is the reward for praying over a child equal to the reward for praying over an adult? And is the reward for praying over a single funeral equal to praying over more than one funeral at the same time?

It is not a condition for obtaining the qirat for departing with the funeral from the deceased's home. Rather, it is obtained merely by praying over it. In Sahih (definition) Muslim, Abu Hurairah reported from the Prophet, peace and blessings be upon him, who said: "Whoever prays over a funeral but does not follow it, will have one qirat; and if he follows it, he will have two qirats." This indicates that praying over the funeral earns the qirat even if one does not follow it.