Hi Boobooyou,
Bit more info needed. Do you mean £70 each or together. I think if the 9 yr old is keen you maybe looking to spend at least £50 on that alone.
Our local skate park, don't encourage under 8's on scooters due to risks of injury, however I feel the 5 yr old will probably still be OK with the cheaper £10-£20 scooters form Argos, Halfords or supermarkets etc.
Few ideas, we are coming to the end of 2012, so like all fashionable things the current scooters are being sold off cheap to make way for the 2013 range. Kids are fickle and they want the next best thing, or improved scooter, so local ads may offer good second-hand scooters for sale.
Don't worry too much about brand names, scooters are pretty much the same quality, although personally I would steer away from Razor.
Two other things to consider, with the bigger lad, is don't let them get into the "my mates got new bars, so I must have them" routine, and if your children develop a keen skill and passion for scootering you will need to replace consumables, such as bearings, handle bar grips, grip tape (on the deck}.
Hope this helps to start with, but I'm happy to give any further guidance if you need it.
Cheers (and Merry Christmas)
Shutz. 