Your beer is sugary? I doubt it as you mean it, some unfermentables but most of the sugar has been converted to alcohol. Bees feed mainly from flowers, so fly towards them and take the line of least resistance and a lot of them are close to the ground.
Bumbles are said to be amazing in that they (well some) should not be able to fly due to their size, weight, power, etc. The bees also need to get rid of items from the nest and some of the low flyers may be taking away refuse. Honey bees will carry away dead bees and can be seen doing this on a regular basis. Probably thousands after the winter rest.
So likely perfectly natural activiy.
They will only sting if trapped, normally. Honey bees die after stinging and so might bumbles, I would not be surprised; with umpteen thousands and a relatively huge food store to protect, honey bees can be defensive. Bumbles don't store much honey, so are less defensive, but will protect the nest if attacked.
Bumbles do not swarm. Swarming is the natural mode of reproduction for honey bees. Bumbles start new nests each spring after the mated queens (from the previous late summer period) have dispersed and hibernated for the witer. They are similar to the 'regular' wasps in this respect.
Bumbles are a threatened species and need all the help thet can get from us - humans are the main cause of their demise, after all, what with extensive use of insecticides for food manufacure, change of use of land, pollution, etc, etc.
Can't remember the last time I was stung by a bumble - even though there is usually a nest, or two, around the garden. Many nests simply go unnoticed because very few are observant or interested in looking for them. Only this week I moved one - simply transferred the bird nesting box to a post in my garden - far better than them being destroyed. The fence, which the box was attached to, was being torn down and the man doing the job was frightened of bees.
If stung, I am sure your very young children will soon learn not to play with them. All part of growing up. But they should not be taught to be frightened of them as they really are not dangerous.
