One answer is that you should say "yes", because it is a federal felony (5 years prison term) to say "no", because it is untrue, and you know it is untrue. This assumes that the question simply asks "Have you ever been convicted or arrested; please explain", with no qualifiers like "as an adult".
If you are absolutely positive that the record was sealed, an alternative answer is "no", based on a law like RCW 13.50.260(6)(a), which say you can legally "act as if it didn't happen":
If the court enters a written order sealing the juvenile court record
pursuant to this section, it shall, subject to RCW 13.50.050(13),
order sealed the official juvenile court record, the social file, and
other records relating to the case as are named in the order.
Thereafter, the proceedings in the case shall be treated as if they
never occurred, and the subject of the records may reply accordingly
to any inquiry about the events, records of which are sealed.
California has a similar law. The problem here is that this is state law, so a valid defense for a state charge of lying, but you need to comply with federal law, and states cannot tell the FBI what to do. It is variously rumoured that the FBI does not report sealed records, but it is unwise to count on rumours, and even if it is general discretionary policy for them to delete information about sealed records when reported from the state, it is not guaranteed that the policy is absolutely always followed. You can request an Identity History Summary Check from the FBI.
So the safest path is to get an informed opinion, tailored to your facts, from your attorney.