Pharaoh responds with a sarcasm that reveals his actual intention.
"He said to them, So may the Word of the Lord be a help to you: (but) how can I release (both) you and your children? The evil offence is in the look of your faces: (you think to go onward) in the way that you would walk, till the time that you shall have come to the house of the place of your habitation" (Targum Pseudo-Jonathan on Exodus 10:10).
The Aramaic paraphrase, preserved in the Targum attributed to Yonatan ben Uzziel, expands what the Hebrew only hints. Pharaoh sees through Moses's request. He understands, finally, that this was never about a three-day feast. Moses and Aaron are planning a full national departure. Ad z'man tei'tun l'beit atar beit m'totevachon — until the time you arrive at the place of your permanent dwelling.
Pharaoh calls this bisha — evil, offense — and declares that the look on their faces is giving away the plan. In other words, he has been paying attention. He knows this is an escape, not a pilgrimage.
But notice what he still refuses to do. He refuses to release the children. A father's heart would know why: children are leverage. As long as Pharaoh holds the children, the fathers and mothers will return.
The Maggid teaches: tyranny always seeks a handhold on the next generation. It is willing to let the parents leave, as long as it can keep the kids. Israel understood then, and understands now: liberation that leaves the children behind is no liberation at all.