The Republic of Heaven

The use of illustrations reminds me of Philip Pullman's shorter stories such as Clockwork and The Firework-Maker's Daughter.

I think Pullman made Lyra's Oxford to be be like this, which fine to make it more accesible to younger readers. However, are younger readers going to read Lyra's Oxford and then move to HDM?

Is that one of Pullman's intentions with Lyra's Oxford?

I don't think so, there's nothing that would appeal more to younger readers than HDM would. I'd say it's aimed just generally at fans of the series; ones that want to know more with the postcard, leaflets etc.

Although its not part of the series it doesn't really stand alone, It wouldn't make sense if you read it without the others.
I agree with ripper, there isn't anything in it thats particually going to drag the children in.