I would not expect a problem. You pay before shipping, they receive their money and will send to whatever address you have registered with Paypal.
I have purchased from Australia, Germany, England, several Asian countries, etc.
The only reluctance may be because you are a new buyer with no Feedback. That makes some sellers nervous, especially if they perceive you are asking for something unusual. However, this could work in your favour as this is your first purchase. So, you simply set your PayPal shipping address as your grandson's address in Oz. You won't need to ask for special shipping accommodation as they will, by rights, ship to that address as it is registered Once the item is complete and shipped, you can change the address back to your own (or wherever you want your purchases to go).
(However, if you are making multiple purchases from different sellers at the same time, and you don't want them all to go to Australia, then this idea becomes a bit tricky.)