If you are shopping for kids bedroom curtains online and wonder when and where should you be placing your order through, then you will not be able to find a single answer that would be good for everyone. If that were true in terms of the fact that a single shop would attract every shopper that goes online, then there would have only really been a single shop or online store.
But the fact that there are hundreds of stores such as http://choicemart.co.uk/home-accessories/bedroom-accessories/childrens-bedroom-accessories/childrens where you could do your shopping from gives away the clue that everyone would have a different preference and liking when it comes to picking a particular store to purchase curtains from. With that said however, it is possible to distinguish good stores from bad ones, as some stores would better be avoided due to lack of customer services.
As for when you should be buying your childrens curtains, it all depends upon you. If you have the budget to do the shopping and you have a need to re-decorate your kids bedrooms, you could well go ahead and do the shopping to acquire curtains appropriate for your needs. But ensure you involve your children when doing your shopping so that you could improve your chances of obtaining something that all would appreciate.