Your child's name won't immediately be changed in official records, even after executing a Deed Poll. As a parent, you may need to change their child's name through the following:
Apply for a new passport for the child, wherein the new name will be automatically reflected in ICA's records;
Update the child's birth certificate by visiting the Registry of Birth. If the child is above one year old, you cannot change the name as it appears on their birth certificate. You can only do so for their ICA records.
Children with an NRIC or Singapore Identity Card (IC) should be able to change their name (with the assistance of both parents or legal guardian/s) by visiting the ICA within 28 days since the Deed Poll was executed.