I'm just a huge proponent of planning. Talk to a real estate agent in your area to find out what homes are in your area are like. How many bedrooms does the average home have? What are some of the more typical floor plans and layouts? You don't ever really want to do something to your house that will make it grander than the neighborhood. Doing that may make it difficult to sell when the times comes to sell it.
Talk to an architect and builder about the costs of having an addition but on your home. Then figure out how you will finance this dream of yours. Find out from your town zoning board or commission if you can build out on your current home or if you will need a variance to do so.
In the meantime and between time, converting a garage into a playroom is a fairly simple proposition but a garage can be a very cold place in the winter and hot in the summer. Converting a basement is feesible but you definitely want windows and a second way of escape and use materials that are floor friendly if possible. Basements flood, even the driest ones.
Personally I prefer the children to be on the same level as me or even higher up. The kids will be 3/5 but they will also become 13/15 and do you want them to have their own basement suite while you and hubby are on another floor?
Long story short, I would probably convert the basement and the garage. I'm all for more living space for the short term but plan and save for the future with the addition onto the house if possible. Dare to dream and make dreams reality. Don't doubt what you can do even in "this" market.