He got up to go, and added: "Then I shall see you in the City before you go back to... Castra Parvulorum, was it? What a jolly name!"Full marks for creativity, but the Latin word would not have developed into the English word in that way. I wonder if Tolkien had something to do with this. (Tolkien and Williams were in the same literary group, along with CS Lewis.)
"Unfortunately it isn't generally called that," the Archdeacon said. "It's called in directories and so on, and by the inhabitants, Fardles. By Grimm's Law."
"Grimm's Law?" Mornington asked, astonished. "Wasn't he the man who wrote the fairy tales for the parvuli? But why did he make a law about it? And why did anyone take any notice?"
"I understand it was something to do with Indo-European sounds," the Archdeacon answered. "The Castra was dropped, and in parvulorum the p became f and the v became d. And Grimm discovered what had happened. But I try and keep the old name as well as I can. It's not far from London. They say Caesar gave it the name because his soldiers caught a lot of British children there, and he sent them back to their own people."
"Then I don't see why Grimm should have interfered," Mornington said
- Charles Williams, War in Heaven, 1930
paruulī are chidren, the dimunitive of paruus "small". It's cognate with Greek παῦρος "little, small", from Proto-Indo-European *peh₂u-.
In English *peh₂u- became few.
Latin paruus made its way into English in parvi- as in parvovirus. And paraffin from parum+affinis, ie "small affinity". It got this name due to its small affinity with other chemicals, apparently.
I don't see a way to get English Fardles and Latin paruulī from the same protoform. If the English was Fartles (**pardu̯-) it might work.