I have no knowledge of the policies and terms involved, but it could very well be seen as being evasive and duplicitous if was subsequently discovered by your stepdaughter's employer.
It might be a bit late now, but I would say WYP would prefer it being declared up front rather than not, given that it is a 30 year old conviction.
On the other hand, she quite possibly would have had to name you as a stepfather on the application form, so they can do checks on close relatives.