Decathlon kids bikes

Looking for a bike for my 9 year old son who is growing quickly so don't want to spend a fortune on something that's only going to last max 18 months, looked at the usual Halfords etc & apart from Carrera they are poor. I have seen decathlon btwin at around £109 & giant mtx at £150, anyone got any experience or advice they could give on either of these please
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I did like the look of the smaller btwin raod bikes though & may look at one of the extra small frames in a year or so & buy online, that way he can get out on the roads for a real blast with me & get away from the wife & his sisters for a few hours