I like the way that, despite one having magic and the other not, you make Dudley and Heather equal - equally interested, equally inarticulate, and equally ready to approach the other when they meet as almost-adults, even though Heather's much more mature than Dudley. (What's new? boys, huh.) And her willingness to share - this line is funny and sweet: "Come on, I'll show you how to fix the playground so you can vandalise it all over again." Given this kind of encouragement, he should take to magic!
The contrast between Dudley's realised hopes and Petunia's memories of loss - loss not just of gentle, shy Ewan, but of her sister - and the lessons she's drawn from it: that's sad. But she wants Dudley to do what makes him happy, and he knows that, so despite her failures she is a loving mother and her love has worked.