The car hire companies insurance will require that your 5 year old uses some kind of seat - more likely a booster seat I would imagine. They are not supplied automatically - the nest advice is to take your own but you can rent them from the car hire companies for something like $5-$6 a day (off the top of my head).

The 7 year old will be fine as they are, unless you feel they need a booster seat.

See the last question on our car hire site FAQ at http://www.uscarhire.co.uk/Faq.aspx for more details.