Terry Pratchett makes up words - words and concepts. I'm afraid I don't remember what 'snickering her nolly' meant, but you might be able to infer it from something previously (or subsequently) mentioned in the book.
The street urchin is described, within a few sentences, as being an adept pickpocket, so I expect snickering means to pickpocket. He was given a hard punch for his efforts by the lady. Nolly may be a word for something unspecified; the nature of which it is not necessary to know.
I'm with MM on this. Pratchett does not usually use variants of anything - he has his own vocabulary. If the boy was a pickpocket, then 'snickering' means picking pockets. I wouldn't be surprised if an explanation of 'nolly' appeared somewhere else in the book.
I also agree (wholeheartedly) with Panj. Definitely a Master.
TP will have made sure that the phrases he used appeared to be allusions to all kinds of other sources. Earlier in that chapter, Vimes and Nobby had a discussion about "street parly" including such terms as whizzing wipers, snitching tinklers, pulling wobblers, flogging tumblers, running rumbles and tottering nevils. Snickering nollies is just another one of those. Any, or all, or none, could be genuine