You need to search Laura. They should have waited and waited till they found a proper cast. Was there any hurry? Look at LOTR! One movie gets 11 Oscars. And our dear old HP. 0 for four.
Think of the money that that would have cost - the countless numbers of auditions they would have had to hold. Its just not worth it. I think they did pretty well, considering. They're hinging whole two hour films on three kids - Lord of the Rings was completely different. Harry Potter is a franchise that has to be cashed in on. The reason they waited for so long with Lord of the Rings is that they didn't have a) the technology, b) a director crazy enough to do it, c) a studio willing to do three films.
With Harry Potter my main problem lies with the casting of Remus and Sirius, both of whom are far too old (and not fit enough!) And also, the way the writers deal with Ron.