Then she tore downstairs and burst into the nursery, crying-- 'He must have wished he was with mother.

He's been there all the time.
"Aggety dag--"' The unusual word was frozen on her lip, as people say in books.
For there, on the floor, lay the carpet, and on the carpet, surrounded by his brothers and by Jane, sat the Lamb.

He had covered his face and clothes with vaseline and violet powder, but he was easily recognizable in spite of this disguise.
'You are right,' said the Phoenix, who was also present; 'it is evident that, as you say, "Aggety dag" is Bosh for "I want to be where my mother is," and so the faithful carpet understood it.' 'But how,' said Anthea, catching up the Lamb and hugging him--'how did he get back here ?' 'Oh,' said the Phoenix, 'I flew to the Psammead and wished that your infant brother were restored to your midst, and immediately it was so.' 'Oh, I am glad, I am glad!' cried Anthea, still hugging the baby.

'Oh, you darling! Shut up, Jane! I don't care HOW much he comes off on me! Cyril! You and Robert roll that carpet up and put it in the beetle-cupboard.

He might say "Aggety dag" again, and it might mean something quite different next time.