The Penderwicks question

I think they should be, just because between the 3rd and 4th book there's a huge time leap that puts the older girls at high school aged and between the 4th and 5th book there's another huge leap that pushes the older girls and Batty into adulthood. So if you read either of those two books and try to go back, things could be very confusing

YES. They MUST be read in order. Not only will it be wildly confusing if you don't, but it's less enjoyable as well.