I think presents on any one year can't be viewed in a vacuum. It's totally fine to spend less as they get older, as long as you intend to spend less as the others get older.
Gifts have a habit of getting smaller and more expensive with age. So a £100 budget (for example) might buy a massive pile of 5/10/15 gifts - everything a 3 year old wants, or a tiny wrapped airpod box for a teen.
I think an adult should also understand less is spent on them as an adult, than the clearly children, younger siblings. But as years go on will anticipate/expect your gift giving to follow the same pattern with the youngers as it did for them.
A different view I suppose, is how do you open presents on the day? If it's a "here's your pile - crack on" then it's not so "obvious" in terms of difference.
If you do, "one person unwraps a gift, then gets the next one for someone else, and so on". Or "everyone takes a gift and unwraps", then that feels different. Even if you know and they know its perfectly fair, it can still feel a bit sad sitting there as its "oh, one for Meg" "One for Sarah" and "Another for Meg" and so on, watching siblings open gift after gift, with only one or two yourself.
And a possibly just me point, but its not so much the value or amount of gifts that make me feel "left out" but the having something to play with on the day. You could buy me some slippers, or an all inclusive spa weekend and the best gift on the day would be something I could use straight away like a puzzle cube.