Storage is vital in these circumstances and cool closets MUST be avoided. Any decent wine merchant will have storage facilities, either of their own or access to shared.
Rioja can be a great wine (despite the vast majority being indifferent) but it's not investment grade and I'm not sure that it would make a good investment. This is important because you don't know what your godchild's interest in wine will be like when turning 18 or 21. Also, do you want it all drunk when the palate is not very sophisticated?
Traditionally, laying down wine always meant Port or fine Bordeaux. Port is very out of fashion nowadays and Bordeaux 2012 was not a vintage to lay down. However, if you chose a really good Cote de Nuits (if your pockets are staggeringly deep) or a really good 2012 southern Rhone (if your pockets are merely deep), they both should have enough life in them and offer a chance of really good appreciation AS LONG AS they are professionally stored throughout their existence, and you can prove it.