you can buy a travelcard but it will be valid for the entire day, no specific timing, but tbh it will be better for you all to get an oyster card each (sometimes there are vending machines for them, but you might have to ask for one at the ticket office). The oyster card will cap at around £5 per day, so that's the maximum you will ever spend with it per day. Are you planning on using the tube much?
Get yourself an oyster card, you can use it on the buses as well. if ya put like £5 on it you'll be fine, and its easy. Look at the map, find where you are currently, then work out if you need to go north, east, south or west and get onto a platform in that direction (signs are same colour as on the map as well).