you know I'm not sure how long it is between SC and LB, but I figure 2-3 years max. Which means that Lucy is 19 in LB. So in LWW which was 4 1/2-5 1/2 years ago, Lucy would have already been 14. Even if you give the full 7 years that some sites say was the length between SC and LB, Lucy is still 10 years old which makes Edmund 11, Susan 14 and Peter 15 in LWW. So by the time LB gets made even with that full 9 1/2 years between LWW and LB, the kids should be Peter-24, Susan 23, Edmund 20, and Lucy 19. That gives even Anna and William and Edmund 6 years to get the films done. And trust me I know plenty of 27 year olds who look 22 so by that time a few extra years won't hurt. And that 7 years isn't definite. I personally believe it was less time because it would seem like more than just Susan would have given up on Narnia. So, the kids could have been even older.
Sorry this is so long, but from what I can tell the actors will fit their parts just fine.