If you know what youre getting them and old enough to drive yourself, id consider taking two vehicles so that when you come out with what youve bought, they dont see. (if its surprise youre aiming for) othee than that, id say just keeping your distance from them and not letting them see the contents of your cart. happy shopping!!
I think you should first research on what you would want to get for each relative. Have a list to help you budget on what you were out to buy. If the relatives tend to find other gifts, impressive but within your budget, you can switch the gifts and have yourself some good time.