Arsenal v Chelsea 3/1/18 – advice on buying seats please


My godson and 2 great-nephews from Australia are all Arsenal fans and longing to see the Chelsea game while they’re here for Christmas and New Year. I’m buying Red memberships for all 4 of us and I’ll link their accounts to mine and I hope to get tickets when they go on sale next week, fingers crossed.
The first alternative is Ticket Exchange and maybe there’ll be some good seats but I’ve heard there’s more seats available with the second alternative, the secondary market. I’ve been offered a good deal by Ticketgum and I’d be inclined to trust Get Me In and Seatwave as they’re owned by Ticketmaster who’ve always been fair when it comes to theatre tickets. I think you get the season ticket or some token of a member who’s sold their seat for one match.
What I’d particularly like to know is whether it’s legitimate for members to sell their seat for one match to companies in the secondary market? If not, is someone at the turnstile going to stop us going in, particularly as my 2 great nephews (8 and 13 years old) will obviously not be the season ticketholder if someone checked on the owner of the season ticket/token they present.
Thanks for your advice, it’ll be very useful.