After the whole murderer escapade near the yard, the fence gets boarded up and it's unclear whether the kids will be able to get into Egypt at all afterward. Even catching a murderer has its downsides.
At the end, the Professor reveals that he's fixed the fence and put in a new lock. But, he's giving all the members of the Egypt Game gang a key so that they can get into the yard and play whenever they want:
The Professor read off the names one by one and handed out the keys. "Elizabeth, Toby, Melanie—" "Is it—is it to Egypt?" Elizabeth whispered as she took her key, and at the Professor's nod there was a storm of comment. (23.33-34)
This is about as positive a storm as it gets. The Professor isn't just giving them a key to his backyard: he's showing them that he cares about their games and wants to give them a safe space to let their imaginations fly. With the key, the kids can go into the yard and turn it into whatever they want—whether it's ancient Egypt or a gypsy caravan. Thanks, Professor.