Overcrowding

Tenancy Types and Management

Our tenants of five years have been very good, looking after our property and paying on the nail.  However, we recently found out quite by chance - through a plumber our agent sent round to make repairs - that the couple we originally rented to now have three young children. They're currently a family of five living in our top floor, one bedroom flat! 
we're currently going down the Section 21 route, but my questions are for when we eventually re-let.

Can we have a clause in the new assured shorthold  agreement stipulating no more than one child? (Given that Section 21 will shortly be 'banned' we need a legitimate means of eviction if overcrowding occurs again).  
If we are allowed to limit the number of children, how can we enforce this? Or even find out about them? Our agent has carried out regular 6-month inspections of the flat over the last 5 years but has never even mentioned the presence of children to us, and as the flat is so small, it never occurred to us to ask!!  It would be easy enough for tenants to hide a cot and take the kids out during an inspection.......

Our present tenants understand they must leave, as once their youngest turns one, the flat will be 'statutorily overcrowded.'  Next time we want to ensure we can stop overcrowding before it starts.

Have others experienced this? Many thanks for any advice!