To find out which magnet is stronger, check: how many paper clips will each magnet pick up? how long a chain will each make? from how far away will each attract? which wins a tug-of-war with a paper clip? how many book pages will each pick up items through? Remember, picking up one magnet with another is no proof of strength - which is doing the work?
Look for evidence that children recognise the importance of changing each factor - the type of sugar, the tea's temperature, the number of stirs - independently. There may be other factors, such as the size, or material, of the cup, or the amount of tea used. Generally, sugar dissolves better if the tea is hot, the particles are small and granny stirs well!