Heading to Isle of Arran early august with jnr ( he's 12) for our first proper bikepacking trip ,anyone got any decent routes ? Only going for weekend

Find the grinduro route?

Not sure how long or short but maybe a starting point?

I'd advise getting an OS map & linking some of these up...
http://www.arranbikeclub.com/routes

It's worth putting in the road miles for a look around the west coast too.

I was there for a week last August and did a few of the Arran bike club routes; they were good but not very technical so potentially just the job for a 12 year old?

If I went again I think I'd take the monstercrosser / a CX bike on which I could ride pretty much everything I rode offroad plus, as deft punk says, a morning or two road riding round the island (which you can split into two 35ish mile rides using the central east / west road).

If your lad likes walking / running don't be put off by the hordes of other tourists, definitely head up Goat Fell. I've done it a few times, but last year was the best yet with my lad aged 3 1/2 managing the entire climb himself (then some strength training with him asleep on my shoulders on the way down!) *proud dad* Running it at 6am a couple of days later was also ace.

Other than getting to the highest point of the island there's better walking routes than Goatfell. Beinn Tarsuinn to the west is a great walk, do the circuit of Coire a Bhradain going anti-clockwise: Beinn a Chliabainn, Beinn Tarsuinn, Ben Nuis. It's rocky but not technical in any way and being the middle ridge of the island you get great views either side.

Have done some of the trails above Lamlash but so long ago that I can't remember much about them, they felt sort of trail centre blue standard. Might be worth seeing if the trails used on the Grinduro event are on-line.

The west coast is nice, even on the flat road. Lots of seals! The South west however, is quite dull (on the road). Blackwaterfoot north is good.

There is a lovely camp spot on the coast north east of Lochranza. Head north on road on the east side of the estuary and keep going as far as you want after the path runs out. Good flat spots with nice views, right by the sea. https://flic.kr/p/W4pwcG