Let us remember that irony is the name given to indicate the discrepancy between appearance and reality. The irony in this instance therefore lies in the way in which Mr. Pocket is a famed and well-known lecturer, said to be the authority on topics such as the management of children, when his own household is such a disaster. His inability to clearly put into practice what he knows so expertly in his head gives rise to a rich irony. Note how Pip describes this situation:
Mr. Pocket was out lecturing; for he was a most delightful lecturer on domestic economy, and his treatises on the management of children and servants were considered the very best text-books on those themes. But Mrs. Pocket was at home, she was in a little difficulty, on account of the baby's having been accommodated with a needle-case to keep him quiet during the unaccountable absence... of Millers.
The way in which Mr. Pocket is described as the leading expert on childhood management whilst his wife has left their baby with a needle case to play with to keep it quiet brings home the full irony of this situation. Pip himself, feeling tempted to share his troubles with Mr. Pocket, takes one look at this situation and decides he would rather not.