One of the oddest feelings is when you come across these series you read as a child -- and there are more books in the series than when you read them. It's actually quite unsettling, I find. While this shouldn't be a problem for a lot of the classic series -- Little House on the Prairie, the Narnia series or the Oz books (more in that series than you might imagine), the jury is still out whether Pullman will add to His Dark Materials trilogy or if Rowling returns to Harry Potter's world.
I was really surprised to learn that Madeleine L'Engle added two books to what had originally been a triology (A Swiftly Tilting Planet, A Wrinkle in Time and A Wind in the Door) with the 4th book (Many Waters) coming 8 years after Wind in the Door, followed two years later by An Acceptable Time. The reviews suggest that Many Waters is a decent addition to the series, but that the 5th book is fairly skip-able.
I was even more surprised to learn that Ursula Le Guin had added three more books to her Earthsea series, with book #4 (Tehanu) coming 18 years after The Farthest Shore and The Other Wind arriving 21 years after Tehanu!
With those gaps, it is pretty hard to put yourself into the mindset of the young teen reader (or even an advanced child reader) that you might have been when the first book came out, since it was basically guaranteed you would be an adult by the publication date of the last one. However, just as there are some advantages to being late adopters of technology, my children will be able to read these series straight through (if they enjoy them), whereas I will have to decide if it really is worth my time to reread the earlier books and then move on to the newer ones. I do recall that the first three in each series are suitable for my son at his current age, and there is something to be said for not having to spend a lot of time pre-screening these books. My son has turned into a voracious reader (possibly reading even more than I did at his age), and it is just too hard for me to pre-screen everything. He might well get through 4 or 5 books in either series in a week. Still, I would be more comfortable letting him read these series, which I have some familiarity with, before the Pullman books, which I think I will want to screen. If I simply do not have the time to get to the Pullman books, then he can read them in a couple more years. I managed to find reasonable used copies of the first four books in each series in these new bundles (still find it weird to see them this way) and ordered them.
I'm pretty sure his sister will be just as interested in reading these books, so might as well make the investment now. Other series I just try to obtain through the library system, but it can be tricky when the available books are out of sequence. I guess it is a mixed blessing that there are so many new series for kids now. Most do not appear to be destined for classic status. However, I was blown away to learn that Stephen Hawking contributed to three children's books (on a SF theme) with his daughter, Lucy Hawking. (Now I feel even more like I am frittering my life away and not actually writing my novel or doing the final edits on my two plays. I wonder if I really have to invest in one of those programs that basically blocks the internet while you work, since I have limited self-control and keep checking for breaking news on 3 or 4 on-line sites.)