Won't be fined.
Here is info from Samut Prakan immigration office.
"Q: What age should my kids start doing 90 day reporting?
ANSWER: If your children live with you and you already do 90 Day reporting yourself, then there is no need for your children to do so too. However, by the time they reach the age of 15, they should start doing 90 Day Reporting themselves. If you child is over 15, there could be a fine of 2,000 Baht"
Quote from this.
http://www.samutprakanimmigration.go.th/frequently-asked-questions/