In episode 12 of The Promised Neverland. Isabella tells Phil, the smartest among the 4 year olds, that the other kids escaped safely and he did not need to worry. Did she know that Phil knows everything about the farm?
And if she did, did she suspect everyone below 4 years old?
Once Isabella understood Ray and Emma's whole plan, she was able to guess that Phil was sent to distract her. Hence why she was fully honest with him : she knew he was aware of the plantation's secret and she had no more reasons to stay deceitful.
Since Phil was just leading around the other younger children to deceive Isabella, she has no reason, on the other hand, to believe they are also aware of the plantation's true nature.