I don't see the harm. I've always had new jammas and slippers in Xmas Eve, way before it was a thing. My mum had it when she was a child too. I like it, it's tradition. When in uni I told my friends about it and one in particular liked it and was going to ask her parents to start doing it.
We didn't have boxes, but there was a routine. Bath with new bubble bath, new pyjamas and dressing gown, fresh bedding. Hot chocolate, book (when smaller, in bed) or film (older) both Xmas themed. Maybe open a small gift that evening, eg a teddy or similar.
So like the Xmas Eve boxes just not wrapped. Jammas, slippers, bubble bath, hot chocolate, film/book, sweets, teddy. When I have kids I'll do the box. Not buy the ready made ones though, extortionate. You can do one cheaper and one more suited to your child's interests, yourself.
I don't think of it as extra 'gifts'. More making Xmas Eve a fun relaxing time. It's stuff lots of families would do anyway. Eg film, hot drink, new pyjamas (ready for Santa). Nothing OTT special. It's just that putting it in a box makes it more exciting for children and part of the Christmas magic.
And I'll continue doing the pyjama thing even when I'm old and gray. Its our lovely Christmas tradition and a part of Christmas
I also don't get the ridicule from some people - the view that anyone who dies Xmas Eve boxes are just spoiling the kids, it's a sign of commercialism gone mad. Or worse that it's a bit greedy and indulgent
. Do what you want at Christmas. Buy your kids a second hand bike or an aeroplane, do boxes or don't, cook dinner or do oven stuff, go to midnight mass or dont. Do what you want. But I really don't get this making fun of looking down on others supposed frivolity because someone chooses to do something you don't agree with.