Penalty for not filing minor child disability benefits

My husband receives disability benefits and as a result our minor children also receive the benefits as well.  They each receive about $3600 a year.  I thought they weren't required to file taxes if they earned less than $11k per year, but I just realized that because it's unearned income they are required to file taxes for income over $1050.  This have been going on for over 3 years.  Any idea what the penalty will be for not including their income on our taxes or for filing their taxes?