It's very funny. It's well written - it has internal cohesion - but it is written in a childlike way. I suspect it was a big hit in its day partly because it includes quite a bit of what could be sexual innuendo (of a gentle kind) made palatable by its innocent expression.
It's interesting that the Preface is by JM Barrie, the author of Peter Pan, which also occupies that confused interface between adult danger and childlike invulnerability. For some time it was suspected that Barrie was the real author of The Young Visiters and I would have thought so myself, but it appears this theory has been well investigated and rejected over the past century.
I am still reading it, but let me quote a couple of places which show you how it is funny, and the gentle innuendo which pervades it.
Mr Salteena was an elderly man of 42 and was quite fond of asking people to stay with him. He had a quite young girl staying with him of 17 called Ethel Monticue. Mr Salteena had wiskers which were very black and twisty.
Mr Salteena's friend Bernard Clark writes to him asking him to come and visit: 'Please bring one of your young ladies, whichever is the prettiest in the face.' Salteena replies.
My Dear Bernard. Certinly I shall come and stay with you next Monday I will bring Ethel Monticue commonly called Miss M. She is very active and pretty. I do hope I shall enjoy myself with you. I am fond of digging in the garden and I am parshial to ladies if they are nice I suppose it is my nature. I am not quite a gentleman but you would hardly notice it but cant be helped anyhow.