Football, especially Premier League, is expensive unfortunately!
Its great that you've been able to find Chelsea vs Man Utd tickets on Viagogo. And they will be sold at the face value - if you bought them through a tout, they would cost hundreds per ticket.
On Viagogo, the tickets there are returns from season ticket holders or other members. And they're only available if you have a membership - so your husband will have to get one too. You can go for the cheapest membership - £25 a year?
If you can wait and get tickets later in December, I would monitor this Ticket News page on the Chelsea website (
http://www.chelseafc.com/tickets). They don't have January games there yet, but they will very soon. You can monitor as they release tickets to season ticket holders, members, and then the general public - and snap them up when they do.
You won't get Chelsea vs Man Utd this way, but you can get premier league games against the less popular teams, as well as potentially Capital One Cup and FA Cup games too (these cup games will be cheaper, at £25 for an adult ticket).
In terms of seating, go for the East Stand Lower as I think this is a special area reserved for only children & parents. While Chelsea fans are normally very well behaved, you sometimes get the odd angry person in the other stands swearing for the entire match.