The general rule is to not use apostrophes in compound words, even when one word ends in -s or -es. Therefore, the correct spelling of the names of rooms is "master bedroom" and "children's room." In such cases, the possessive form is implied without an apostrophe.
However, there are a few exceptions to this rule. For instance, if the room has a name derived from a person's name (e.g., "Sarah's room"), or if the room is designed for a specific person or group of people (e.g., "the baby's room").