Other mistake: When Oliver is meeting Fagin and his boys, he says, "Is this a laundry then, sir?" You can see Jack Wild (Dodger) mouthing that line on the left side of the screen.
Other mistake: At Fagin's place, Bill is supposed to give Nancy a back-handed slap that sends her to the floor, but there's no sound of the slap and there's no contact - it's very badly acted.
Chosen answer: They're probably concerned because he's a child. He might unintentionally say something without meaning to, or after some time has passed and more is learned about Oliver's past life at Fagin's hideout, it's conceivable he might be questioned more intensely. A child would likely give more information under pressure.
raywest ★