I like Board-O's advice. If you can, pick a corner that doesn't get morning sunlight. If you can, great; if not, all is not lost.
Build a frame at the corner with 2x4s, board up, add a vapour barrier, insulate. If part of the basement is above ground level, your cellar drop ceiling should be slightly below ground level. Add an (insulated) door. I'm no carpenter but this is easy stuff - a friend or handyman could build this in about 4 hours.
If your temp crawls above 15C in the summer, you can always cut out a hole, and add a VK or other chiller unit (starting around $500) between the wall studs.
With a locked door, you have a good amount of security. Plus, there are thermo/hygrometers on the market that work with an iPhone app (or give you extranet access) and that relay temp & humidity data in real-time (you need WiFi for this, naturally) - so you can have peace of mind that your babies are sleeping well.