Make sure that you dont make them too hard to get in to - the last thing that you want if you are under some stress on a dark night on the roads. Gaffa tape is damn useful stuff though.
Realistically, you are going to have to be thinking "improvisation" with some of the items in your smaller kit. You can happily substitute staple items such as triangular bandages with other things that you might have around you - probably saving you half of the space that you have in the smaller kit.
I am assuming that you arent going to be too far from help. If it were for me, I would be thinking about things like eyewash, an eye pad (you can use something else to cover the good eye), some micropore or zinc oxide tape, a number 8 dressing and maybe some non-adhesive dressings and some adhesive dressings. A small decent pair of scissors is always a bonus - especially if you are going to be looking for things to cut up if you have slimmed down on the kit. AT least 2 pairs of gloves - too many first aid kits dont have 2 in them and that just isn't very sensible - what if you tear one or have to deal with more than one bleeder. A slimline (non plastic valve) resus aid is also a good idea. Add to that lot a bit of waterproof paper and a pencil and I think you are there.
I don't know your level of training and / or experience or what else you might have in your car. I won't go in to meds - but you know what you usually have to give your kids or treat yourself with.
For the kids - I would probably have something along the same lines - with probably double up, but think about adding things like small cold packs and some clingfilm - loads of uses there.
Probably loads more suggestions to come.
