If your happy enough and have time to wet the seats and let them dry, scrubbing them with a soft scrubbing brush and warm soapy water works a treat. Also great for the car mats and inside doors.
I would have once recommended the same but now am vehemently opposed to it. There is nothing here that cannot be done yourself, using a combination of elbow grease and the right materials. I recently valeted my own car and did a perfect job. Last year I paid a guy €150 to do it. Never again.
got an aerosol car in Halfords, bog standard upholstry cleaner, nothing swanky. Using it and a (clean) washing up brush I scrubbed the seats and they came out great, light coloured fabric with all manner of juice, bikkies, chocolate, yoghurt stains (I'll get those darned kids ...as they say in Scooby Doo !!).
New buyer said the car was immaculate (as it was). Will post the name of the cleaner tomorrow but a DIY job is more than sufficient.