My understanding is that provided your children have TFN's and earn a taxable income, they would qualify for the co-contribution scheme.
For example, I have two school-age children who have part-time jobs and probably earn about $1,000 to $2,000 annually. The co-contribution scheme is certainly working for them i.e. for each child we put in $1,000 and the ATO puts in $1,500 each year! Not a bad return and arguably all the better if you can include them under your SMSF. Same applies for a Uni student who could, of course, be earning a lot more than $2,000 a year part-time.