It is not made definitively evident in the novel the exact ages of the three Drew children. However, we can make some guesses as to their approximate ages by what they say and do.
Simon is the oldest; Jan is the middle child while Barney is the youngest. As the youngest, Barney seems to be very adept at reading people and making inferences from curious situations. It is he that makes that name association between his uncle and the magical Merlin. His age could, thus, be no younger than 10 years or so, making Jane closer to 12 and Simon around 14. This fits, as Simon is naturally cautious, and Barney naturally effusive.
These are estimates based upon Wagner's review (attached) that points out the dialogue by the children is natural to their age groups.