I just had a quick look at the two books you mentioned in particular - Mandrake's plot and Martha and Mitch - the covers aren't great - they give me no insight as to what the books are about or the audience you are targeting. In particular Martha and Mitch you can't really read anything due to the colours. I'd also question the title, make it a bit more fun, it could be subtitled a Martha and Mitch adventure but the title doesn't really say much to me.
As for the blurbs they seem fine, the only thing I would highlight is the target audience. Headline the blurb with A fun adventure for age 9 to 12's or whatever the age you are targeting is.