There are quite a number of options by rail if you aren't going on the supporters coach, with prices depending on whether you have a Family Travelcard, and whether you want to start from Wellingborough or Bedford. There are also "Groupsave" options for groups of adults (with or without children)
As a startpoint for a more detailed review, from Bedford you can get the Super Off Peak Travelcard for £20 adults, (and I think this allows up to 4 children at £2 each - see the link below). This will get you to the ground - either via Norbiton (and walking), or Kingston (and then a bus - with more options for shopping beforehand) - I did both options last season.
Unfortunately you also need to check which tubes aren't running - but the Victoria line looks OK this weekend, so your ticket will get you to Vauxhall where you can pick up a Kingston train.
There is some debate about whether you can use the Super Off Peak Tickets on East Midlands Trains from Bedford (as opposed to First Capital Connect) - I know a few people were told you couldn't last season, and there was a notice to that effect at Bedford - but all the travel booking sites offer these prices against the faster East Midlands Trains.
http://www.firstcapitalconnect.co.uk/ti ... -off-peak/
http://www.afcwimbledon.co.uk/location. ... d=9&squad=
http://www.tfl.gov.uk/tfl/livetravelnew ... et=weekend
http://www.southwesttrains.co.uk/networkmap.aspx#