Say it on the invitation
Address each invitation to exactly who is invited. 'Mr and Mrs Patel' invites two; 'The Patel family' invites the children too.
Let the RSVP do the work
With household links, each family's RSVP allows the number of guests you set, so nobody adds a surprise plus-one.
Children
If the wedding is adults-only, say so gently on the website's FAQ page, and thank parents for understanding.
Every guest, beautifully answered
Begin with a design, or start free with the basics. Your link: rsvpea.co.uk/your-names.