Just give DCL a call (or ask your travel agent to do it if you have booked through one) and ask them to note a request. There are also other things you can request, like a certain table size (it is common that more than one family will be placed at a table, depending on your preferences you might want to make sure that you get a shared table or that you have a table just for your group) as well as a specific restaurant on a specific night. However, the general recommendation is to make only one request as otherwise things might not work out.
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