Parents were not allowed to visit the LA preschool due to strict rules.
The royals made a trip to the Assistance League Preschool Learning Centre wearing face masks and planted forget-me-nots in memory of Princess Diana.
The visit marked the 23rd anniversary of her death, where they took part in digging and planting for at risk children in LA.
However, parents were not impressed that the duo were allowed to enter despite coronavirus restrictions.
One user on social media commented: "I could not even hold my daughter's hand and take her into school last week when she started kindergarten. These two just 'turn up and break the rules' whenever they feel like it?"
Another added: "Not safe to go back to school but safe for mingling with strangers for a photo op."
One expressed their disappointment and said: "Exactly I can't take a pencil to my child right now. NO visitors. No exceptions.
"They have no respect for the rules as they are 'humanitarians' they are the exception? BS kids in the US have no clue what a Duke/Duchess even is but they can come in when I can't..for a pic?!?"
It has been reported the Los Angeles County Department of Public Health issued strict guidelines for preschools.
The guidance read: "Modify daily operations to minimize exposures (e.g. avoid the use of touchscreens for sign-in/out, ask parents to bring their own pens to sign in, implement valet services or have staff meet children as they arrive to avoid parents coming into the childcare center)."
The pre-school learning centre provides childcare to youngsters aged three to five from low-income families.
Taking to Instagram, the Assistance League of Los Angeles wrote alongside the sweet snaps: "The children of our Preschool Learning Centre got a wonderful surprise when The Duke and Duchess of Sussex visited yesterday!
"In addition to helping the children replant the Preschool Learning Centre's garden, they spent time with them, sharing their appreciation for nature and helping to instil the importance of healthy eating. We truly appreciate their time and care for our students."