I wore a white sleeveless top (they were not āspaghetti strapā, they were thick) and a long white skirt for my holy communion, the church sold white outfits for us a few weeks before the event. So Iām sure sleeveless is allowed. However I know you might be personally against that or your church is different or something like that.
Must it be a dress? Because itās easy to find a long skirt and a white top but itās way more difficult to find a nice dress that doesnāt show too much.
Honestly as it is spring/summer, long dresses are easy to find, but they are usually too revealing at the top, so maybe a camisole underneath would help, or you could sew extra fabric. It wonāt look terrible. If sleeves are the issue, like others said, a light cardigan would help.
As for weekly mass, again, must it be dresses? Skirts or pants would be a lot cheaper as you could mix and match with different tops. My sister wore dresses and we found she kept wearing the same clothes, which made her despise the whole modesty thing. She is 11 and tall too! I usually wear pants to mass but i know people here donāt agree with that

But anyway, say you want dresses, cotton on, h&m, etc have modest dresses, you just want to buy a size larger so the length is appropriate enough for you. The neckline and the sleeves are always easy to correct, but the length is tricky. I would say maxi dresses and skirts are your best bet. H&M does midi skirts too, although Iām not sure your daughter could fit them