Looking for a graphic novel(s), roughly mystery stories targeted at high primary/ low secondary readers. Would have been published by 2007 in english. Art style is not particularly unique - but it is 'western' - by which I mean distinctly not chinese or japanese. One features a lighthouse and a painting of a possible witch(?) who causes storms, it is ultimately unclear at the end of the novel whether the storms have a natural or supernatural cause. The other is in a similar style, both artistically and story wise, it involves a calculator and a theatre(?).
In both, it is a group of children who are thrust into the events of the mystery.