Practical side - food and drink is limited and expensive/big queues, so I'd take your own
Not many people like the butterbeer but they do like the ice cream, so I suggest going for that, and for a small amount extra it comes in a souvenir bowl instead of a cone and you can take that home (and they have washing stations for them) which is a much cheaper souvenir than anything from the shop, and can be used for ice cream at home to bring the memories back
I ordered Harry potter sweets off Amazon before we went, and gave them a little goody bag halfway - it meant they weren't interested in wasting their money on the sweets and chocolate frogs which are really over priced. The rest of the giftshop depends on what they are looking at, there are some pocket money souvenirs but there are also wands for £29.99 etc - I'd buy the wands from eBay, my friend warned us that she bought them there and they broke within days and the DC hadn't been silly with them, she ordered replacements on eBay for under £10 and they looked exactly the same in the silk lined box and everything, 3 years later they are still in one piece and ours from eBay are 2 years later
If you see anything at the shop by the train, it may be worth buying it then as not everything there is in the main shop which I really don't like as they are basically forcing you to buy without being able to compare, so that you will then see something in the main gift shop and buy that as well instead of choosing between the two
If they have costumes then wear them, you will see plenty of DC and adults walking around in them