I use a booster seat in the car and in the van. Basically all three of my kids have used it as they've grown up and it is a bumshaped (

) plastic booster with a thin bit of cushioning on the top.
It lifts them to the correct seatbelt height and they can see out the window better. My 8 yr old still uses it.
If they are toddlers (you know - large head in proportion to body) then perhaps a full child seat with back and or sides. But once mine got to about 4/5 they just used the booster. The shop will advise but make sure the belt doesn't cut into the neck - most front belts allow up and down movement for the pivot point.