Aw man just seen those Zara ones! They are so similar to what I have been envisaging, i love them! My ds did have some cute ones from zara but I went in the other day and there wasn't one pair, all chinos and jeans.
And I must be the worlds worst googler, as I have been finding them difficult to get hold of.
I have just realised the leggings I am interested in purchasing are a ribbed type of material so probably not the best to be printing patterns onto, but are a brilliant quality. So might start with the plain ones and concentrate the prints and motifs on the tshirts.
Thanks for all the responses, it really helpful and its nice to hear of so many parents dressing their sons in leggings. I found a thread from a couple of years ago with someone asking about them and a lot of people gave some withering answers about sticking to 'normal' trousers.
Im aware designing and making the products is the easy part, its the selling that is hard, but I am lucky we have two fantastic vintagey 'hip' (sorry :D) markets that I can test the waters with.