The portal that opened into Hueco Mundo was not one by either shinigami or arrancar. It had originated in Urahara Kisuke's shop, and the man and the boy of barely 4 years that stepped out of it were both human, somewhat. Aizen had of course known immediately that someone was within his domain, and he also knew exactly who it was. What he didn't know was why they were there. So, he sent his cuatra and sexta espadas to see what was going on, with explicit orders not to harm either person.
Ulquiorra and Grimmjow were standing in front of the man and child within moments, both perfectly calm and ready to show the utmost hospitality and kindness as per Aizen-sama's orders. "Isshin Kurosaki," Ulquiorra said with his usual emotionless voice, "Why have you come here?"
"I have come to speak with Aizen, Gin, and Tousen. I am in desperate need of their help." Isshin replied, picking up the child as he did so.
"Very well, but before we escort you to Aizen-sama, I wish to know one thing." Ulquiorra drawled.
"Yea, why the hell do you have a kid with you? I thought all your brats were teenagers." Grimmjow snapped.
The tone of Grimmjow's voice made the child shiver and bury his face into Isshin's neck.
"Oh, don't you recognize him? This is my son, Ichigo."