Any of the vendors or shops will quite happily give you change. I know inside the Emporium in Magic Kingdom - there is a machine that will change dollar bills for 4 x 25 cents. You will have no problem in getting change
In the contemporary resort there are pressed penny machines near the restrooms on the level where the gift shop is and there is actually a machine that will give you change in quarters. We have never needed this. We find you tend to accumulate change way too quickly there without even trying if you buy things in cash at any time. But as Nigel says, anywhere with a cash register will have a smiling CM willing to help you out with whatever change you need
We ordered a couple of baskets one year from Memoriesbybetsy.com and she was able to put each child two roles of coins in their basket together with a pressed penny album so that they could start right away - I'm not sure if anyone else does this.
You can also download a list of all the machines from allears so the kids can mark them off each year as their collection grows.
off topic - we found a pressed penny machine at the National Rail Museum in York in the holidays - such excitement lol
You can also buy the pressed penny wallets at most of the gift shops for then to keep their pennies in. The first year we did it they were all thrown loose into my purse and we ended up with several duplicates. My DD derived great pleasure from examining and sorting her coins in her wallet every night once we got that. I think we paid around $7 for hers a few years ago and they seemed to be the same price last year when we saw them. Although they add up, it is still one of the cheapest souvenirs to collect around Disney and if your kids are younger or at any age, prone to getting cranky when tired, it serves as a great distraction from irritability that starts to develop. Enjoy. Like most collections, it becomes slightly addictive
Another one that is good for the kids to collect are the quarters that have the names of the states on the back of them.
There are actually 2 distinct collections available - though I will be honest and say I wouldn't know the difference between the two.
The last 3 trips of ours have resulted in DS checking every single quarter we got to see if any of them had the states on the back of them - even though he already had most of them after the first trip.