Our 6 year old asked the same thing when we got Charlie - and also asked if maybe Charlie needed a friend. I just explained to him that Hamsters aren't like humans - they need to live on their own or they would fight and kill each other. But that hamsters make good friends with humans. I think it's nice that kids worry about a hamster being lonely and needing a friend
Or someone to play with - but it's all part of them learning about hamsters.
I have heard of the rare story of a hamster being friends with another pet - a dog I think it was - but it may just have been a very tame hamster and who knows what the hamster thought! You can't see the stress that races inside them sometimes.
I don't know much about gerbils but as Hamsters are so territorial I think anyone in their space would be a threat and it could lead to fighting and injuries - but more than that it would be very stressful for the hamster.
They are like little hermits, Syrian hamsters. What is lovely though is how they can bond with an owner. Although I say "hamsters" above (dwarf hamsters can live together but it often ends in separation after fighting and only works with siblings really even if it does last for life).
With the gerbils there is also the issue of diseases. A micro-organism that may live quite happily on one creature could cause illness in another. In particular Hamsters can get a very unpleasant condition from wild mice - Tyzer's disease.