The younger can get a non-O piggy-backing on the others non-O. And should they get an extension to stay long term can do the same on that also. The older should get their non-o first and the younger shortly after, as the olders visa will be needed for the application.
Non-Immigrant O (Visiting Non-Thai Family Residing in Thailand)
The following documents must be uploaded in the E-visa system in large and clear JPEG or PDF format (the file size must not exceed 3MB):
Passport biodata page or travel document (must be valid within 6 months from travel date)
Photograph of the applicant, taken within the past six months
Document indicating current location (driver's license, bank statement, or proof of stay)
A copy of a recent savings or checking bank statement showing an ending balance of at least $1,000 per person or $2,000 per family, with the applicant's name and date. If submitting a family bank statement (joint account), proof of relationship (e.g., birth certificate, marriage certificate) must be provided
Personal details of a non-Thai family member in Thailand, such as a valid government-issued ID card, passport with Thai visa page, work permit, or stay permit in Thailand
Proof of relationship to a non-Thai family member in Thailand (immediate family only), such as a copy of the marriage certificate, birth certificate, or adoption certificate