There is no such thing as a stupid question.

When Alexander died in 323 BCE, his son with the Bactrian princess Roxanne was not yet born. The person who was right there and waiting to take control was Perdikkas, Alexander's close friend and supporter. Alex supposedly gave him the signet ring upon his death, therefore signifying his intent. The army (in assembly) supported Perdikkas, but he was slow to take the reins (perhaps because he respected the claim of the unborn son). Alexander may have fathered a son named Herakles with the Iranian Barsine, but this kid's claim was quickly ignored. While Perdikkas delayed, the mentally-disabled half-brother of Alexander, Philip Arrhidaios, showed up and got support from the army.

Meanwhile, one of the generals, Ptolemy, proposed that rule be established among a council of the most influential players. This idea ignored blood relatives, and was perhaps not so strange: the right to rule in Macedonia had always been something of a violent decision, frequently solved in armed conflict rather than by claims of blood. When a Macedonian king died, the strongest of the aristocrats were always there to cause trouble, and in this case it was no different. In those earlier days, the prize had been regal domination of ancestral Macedonia; now, the stakes were much higher: Alexander had cut a path from Greece to Egypt to the Indian frontiers, toppled an Empire, and just in general created a huge mess. He then died before anything could be cleaned up, clarified, institutionalized.

After his death, the generals (as you say) wasted no time in moving in on the kingship. The son (Alexander IV) was eventually born, but was never anything more than a pawn. There would be many deaths (including Perdikkas and another powerful individual, Krateros) before the dust finally cleared.

A great resource for the Succession is a short account by Braund ("After Alexander: The Emergence of the Hellenistic World 323 - 281 BC" in the volume A Companion to the Hellenistic World, edited by Erksine.

It's not a stupid question at all. The series of events following Alexander's death is so convoluted it's very easy to get confused.

When Alexander died in June 323 his son was a few months away from being born so no one knew if the child would turn out to be eligible for the succession. With no son at the the time there was a great deal of debate over just what the future of the monarchy would be. One faction within the Macedonian leadership, headed by Ptolemy, favored doing away with the need for a king altogether and rule the empire under the rule of a Macedonian junta. A second element argued that the most senior general - Perdiccas - should rule as regent while the birth of Alexander's child was awaited. Both these proposals gave the impression that the generals weren't committed to preserving the monarchy so a third group arose demanding the installation of Alexander's half brother Philip. The men backing this idea were the common men of the infantry who felt a much stronger sense of loyalty to the royal family than the generals did.

The generals didn't really want Philip because he was, to put it bluntly, mentally handicapped. Just what afflicted him is unknown, but it had been enough to sideline him for most of his life. But for the infantry, a weak minded king was better than no king at all. There may have also been some prejudice against the prospect of Alexander's unborn child taking the throne on account of his mother, Roxanne, being Bactrian (i.e not Macedonian or Greek).

A slapdash compromise was reached whereby Perdiccas would rule the empire as regent for Philip, with Meleager (the infantry leader) as his deputy. If Roxanne's child was male, then he would join Philip as a joint king under the Perdiccas regency. Babylon was to be the capital. This idea was tentatively agreed to and Roxanne did eventually give birth to Alexander IV.

This rather unorthodox arrangement didn't really work out. Perdiccas died two years later in a civil war and the kings were taken back to Macedon to become pawns in the dynastic power game. Philip fell victim to the coup of Olympias (Alexander's mother) who had him executed in 317, a crime for which she in turn was killed. Alexander IV and Roxanne ended up in the hands of Cassander, a man who had no interest in maintaining the monarchy. He put them under house arrest and had them killed about 10 years later.

None of the generals really had much interest in serving the monarchy and only feigned support for it when it suited their interests. Alexander IV was a boy king who existed only to be manipulated by those who sought to build their own kingdoms out of the wreckage of the one his father had built.

Follow up question - I've read Ghost on the Throne by James S. Romm and found it enjoyable and educational. What are the thoughts of some of the experts on this book?