Sat Navs: Allow for anything up to £50 to £150 for a one off rental of a sat nav. If you can't beg or borrow one, it's probably better to buy one (from around £80) otherwise it's dead money. You also risk paying a premium to hire one and then being not able to change the language from Germany's equivalent of Joanna Lumley. 'Links' & 'rechts' anyone?!
Child Seats: Now, this is where it hurts particularly if you have a brood of pre-schoolers. Be prepared to pay around the £60 mark for an infant / toddler seat and around £25 for a booster seat.
I always find child seats a bit like hotel rooms. You never like the first one so you have to see a couple before settling on one. (Perhaps I should explain that this is generally my other half's prerogative while I sit with a resigned air on the suitcases in the hotel lobby.)
Once you've accepted 'the seat' (now you've got the correct one for the age of the child / it's not covered in marks [this is a euphemism!] or is so tatty you don't want to put your child in it), then prepare for the half hour ritual of swearing and arguing with your other half as you can't fit the damn thing despite prising the written instructions from the car rental desk. By the way you'll get no help to fit the seat for "insurance" reasons.
DVD Player: This might seem like a brilliant idea especially to keep the kids amused, but again you are probably adding another £50 ish to your bill. Buy one instead for around the £100 mark.
Roof Rack: Just to top off your bill, this will be another (roughly) £15 "please sir (or madam)".
KERCHING - Two young children, with hired sat nav, DVD player and roof rack and that's a whooping £281. Ouch!
Final Point: We're assuming you haven't forgotten to buy your excess car hire insurance with iCarhireinsurance.com 'before you fly' to get a more comprehensive policy and at up to 70% cheaper than you'll pay at the rental desk.
NO, we thought not!