These are the two things i think can be the easiest/most effective ways to support dcs with no cost.
Make use of car journeys - find appropriate pod casts for them to listen to on the commute to school - get a chief quizzer for the car (revision guides have questions in them) then ask questions when you are going somewhere. They feel like it isn't taking any time from them as the journey would be happening anyway.
The best thing you can do is download the spec for each GCSE they studying and ask them to explain the bullet points to you based on which units (sub topics) they have studied so far. If there is something on the spec they are unclear of then help them to get better at it. Of course you wont be an expert but you can a) google b) get a feel for areas of weakness.