I can't really help you in regards of what to expect with babies/children being diagnosed with an arachnoid cyst, because my cyst wasn't discovered until I was an adult.
I did have some learning difficulties and co-ordination/balance problems in my early years, but in those days (40+ years ago) they didn't know the cause of them. But when my cyst was discovered (with MRI) I was also diagnosed with Dandy-Walker Syndrome (a mild variant). I had fenestration, and never shunted, so I don't know what problems could occur there. Does this child just have a cyst, or do they have Dandy-Walker or Chiari malformation as well? They are usually associated with learning delays and co-ordination/balance problems...