Aggressive dog in certain situations

especially children on bikes

and

The child never did anything to upset him and they got along very well

and also

He gets along with other dogs in general

These two statements makes me think that the problem are not children but... bikes. You should know already that dogs chase bicycles.

It is not because dogs hate bicycles, but because they are "alerted" by the ultrasounds created by the said bicycles.

Ultrasounds are sounds which have a very high frequency - a frequency so high that you cannot hear anything, but the dog can hear very loud. As a consequence, the dogs will react in various ways: with fear, alertness, pain, aggression... - it depends on a number of factors, like sound frequency, sound intensity, the environment, dog's predisposition...

Please note that other things / mechanisms create ultrasounds, which could trigger your dog, cars included, as well as any number of toys.

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we are getting him a sister in a couple weeks since he is pretty high energy and loves running around with other dogs with every new dog he meets

Some (all?) dogs are very sociable, so getting him a partner can be a very good choice.

From your description alone I think it's impossible to know what emotion causes your dog's behavior. It could be any of:

• Excitement (wanting to play) triggered by the movement
• Anxiety or insecurity that turns into aggressive behavior
• Herding instinct, also triggered by the movement (don't forget that corgies were bred as herding and guard dogs)
• Guarding behavior (protecting you from the chaotic children)
• Dominant / macho behavior (especially when you pick him up)
• Fear, caused by a lack of trust when you pick him up

You see, there are many possibilities, maybe even more that I didn't think about. Without knowing the real cause and motivation of his behavior, you cannot train to stop it. Unfortunately, the only advice I can give you in this situation is using the service of a professional dog trainer. Not the puppy school type, but someone who also trains problematic dogs. These are the only ones I trust to know and analyze a dog's body language and recognize the motivation behind certain actions.

As to getting a second dog, you have to be really, totally, brutally honest with yourself:

Did you train your current dog well enough that you have him under control most of the time? Or do you hope that a second dog would magically calm his chaotic behavior and aggression down to an acceptable level?

If you answered the first question with "yes", then you know how much effort it is to train a puppy until it behaves in a civilized way. Expect to invest the same amount of effort into the second dog.

If you answered the second question with "yes", then I strongly recommend you think your plan over. Just like children, puppies copy behavior from the older people / dogs around them, especially unwanted behavior. If your first dog is aggressive, disrespectful or uncontrollable, the second dog will pick this behavior up from him. Both of them will then goad each other, amplifying the worst of their behavior. Before you get a second dog, you must make sure that the first one behaves well enough.