There's isn't a way to notify you of the item being restock in size 8, but normally, these would be restocked in the spring. The pandemic has affected replenishment of a many items, unfortunately.
Consider some of these in Size 8:Roebuck & Co. Girls' Occasion Dress - PlaidNanette Girls' Embroidered Occasion Dress - FloralNanette Girls' Brocade Occasion DressChildren's Apparel Girls’ 2-Pack Skater Dresses Set
You can also consider girl's dresses in Small, which according to the Size Guide are equal: sears.com/en_us/sizechart/apparel/standard/girl/sizechart_sears_girls_416.html
Here are Girl's Dresses in Small or Medium: sears.com/clothing-girls-clothing-girls-dresses/b-5000970?Size=M&Size=S&filterList=Size&shipOrDelivery=true&searsTab=true
Hope this helps!