I actually have this issue with some of my kids. Baruch Hashem lots of hand me downs from older siblings from different seasons but the oldest of each gender is getting new clothing. Especially my dd who’s 8 and her skirt is getting short. I’m not into spoiling either but I think the chinuch in tznius is important and I learned my lesson for next time to just get it longer. Maybe hem it by seamstress then open (for shabbos clothing) 🤷♀️ idk just an idea for myself. I have taken a dress and turned it into a skirt for dd so she could still match rest of siblings
It's seems like very normal growth. You can buy bigger at the beginning of the season. Or add on fabric to the hem of the dress.
I find buying bigger sizes doesn't guarantee that the clothes will last until the end of winter. Kids clothes get ruined in other ways too. And you just have them look baggy for a few months and then the dress needs to be chucked anyway.
But do what works for you. And you're not spoiling them. Just taking care of their basic needs.