Hi
What type of cage is it? Wired or plastic? If it is wired a lot of them aren't suitable for dwarves as they can squeeze themselves down and try and get out.
There are fors and againsts of both syrians and dwarves, syrians are easier to handle due to their size and their slowness (not sure even a word!) they are more likely to be petted, however if they bite then it is a lot more painful and brings out a lot more blood than dwarves, dwarves can be faster and can sometimes not take well to being held.
There are three main types of dwarves: russians, chinese and roborovski (listed in terms of speed from slowest to fastest) I would definately stay away from the last two but russians can be fine, they stay very small but tend to go rounder rather than longer, if you did want to get two they often come in different colours so you could choose two different ones however they are known to fight with each other sometimes as well which would mean having to seperate them.
Dwarves tend to live slightly longer and do not suffer with "wet tail" as much as syrian hamsters do (although they can still get it).
As your DS is four please bear in mind hamsters are nocturnal and there is a chance he won't see a great deal of them as he will be asleep so it may be a case of waking it up near his bedtime and letting it aclimitise before he gets it out, if not happy at being woken up it may bite. This is the only downside with hamsters and small children so I would normally recommend guinea pigs at this age, although you'll be doing all the cleaning!!!
Best of luck, keep us updated as to what you decide, if you go to any good pet shop they will let you see the full range and help you decide but have a read of the intrnet first to see other people's stories!