It's not like there is an international baby stroller organization setting standards for this. Any grade could be used, you just need to select the section properties appropriately.
Being how strollers need to be cheap and fairly light (they get heaved around a lot) I'd have thought 6063 wasn't a bad starting point. It should keep tubing, etc., a little lighter than a beer-can type 3000 series, and for a stroller the corrosion resistance of 6000 series should be adequate.
