A Care Quality Commission spokesman said an unannounced inspection at Basildon and Thurrock University Hospitals NHS Foundation Trust in Essex was prompted by "a number of serious incidents involving the care of children at the trust".
"Staff said there were often considerable delays in children receiving medical attention when concerns over their condition deteriorating were escalated to doctors, particularly after 5pm and at weekends," the spokesman added.
It is not the first time the trust has been issued a warning by the health watchdog. Earlier this year it was ordered to make improvements following inspections of the accident and emergency department and adult wards.
The health watchdog has "serious concerns" about the care of children at a scandal-hit hospital.
The Care Quality Commission (CQC) said "rapid and sustainable improvement" is needed at Basildon and Thurrock University Hospitals NHS Foundation Trust in Essex.
The watchdog has also ordered an independent investigation into the paediatric services provided by the trust. During an unannounced inspection earlier this month, the CQC noted that there were problems with waiting times, the numbers of senior staff and out-of-date medicines.
"The trust was found to be failing to plan and deliver care to meet the needs of children in a way that ensured their welfare and safety," a spokesman said.