The Model B Com I used to fly had a rear bench seat and one of the lap straps was very, very long.
It was possible to thread it through the other buckle and hence restrain 3 small peeps.
By careful attention to the weght and balance charts, and weighing the little ones, it was feasible to fly with 3 in the back.
I know in these health and safety, risk assesment, litiginous days this must seem heinous, but I used to ride a bike without any form of protection too!
No one was harmed in the writing of this reply.
Sir George Cayley
ps Thanks for the reminder about the single Com. Whata playne!