I was recently asked this question by a couple who is in the midst of wedding planning. If there is a ring bearer and a flower tosser (we won't assume it's a girl) who pays for their attire?
Just like the wedding party, agreeing to be part of your big day means agreeing to pay your way, which includes the get up. The parent's of the kids will most likely be aware that they need to provide the garb, however, this is something that you can discuss when you ask for their child's participation.
If you are planning to pay for your wedding party's attire, you should also include the tyke's clothing!