I am reading The Scarlet Pimpernel for the first time and the author (writing in the late 19th century) is constantly referring to children being executed. Were they really putting 8 year olds to the maws of Madame Guillotine? Or is it a case of children as we'd see them now but adults then, like 15 year olds? (Googling "reign of terror executions children" naturally produces articles on the Reign of Terror for children, apparently a big market for this.)
An interesting question! It will depend somewhat on what the boundaries of the reign of terror are (eg - do we include the Vendee war in there?). In the Vendee, women and children were certainly executed in thousands, but it was a much more bloody and reprisal filled part of the French Revolution. They were also almost entirely of the lower classes, not aristocratic.
Our image of the reign of terror tends to align more with what we see in the capital, and that seems to be what your question is asking about. That's where it gets a little more iffy to find - some digging doesn't really find much on children being executed (certainly I haven't found anything about 8 year olds being guillotined).
The closest I stumbled across is here, from a memoire of a French noble/politician. On page 119 he talks about part of the Terror, and how his brother was imprisoned but had his name stricken off the final list of executed by one of the jailers, which saved him - unlike one of the others imprisoned with him. The relevant part to age is that Pasquier describes his brother as the oldest of the group imprisoned, and it seems he was 19-20 at the time. The younger man executed, a de Maille, is also noted to have been too young for the death penalty to be applied. I don't seem to be able to find great sources on his age, but he seems to have been roughly 16 years old.
Later than the revolution, there were a number of who we'd consider children guillotined. Those seem to be easier to find sources on - eg, Albert Jeantroux in 1890 (age 17) or Jean Dubois in 1840 (age 17).