Listen to your instincts – they sound good… if you weren’t worried you’d be showing poor judgement IMO.
That route is 2/3 of the Welsh 3000’s which is a serious proposition for experienced people (I’m told most people fail on their first attempt)
Tryfan doesn’t have a walking path like Snowdon – so the nav and the going underfoot are challenging for a novice, let alone with everything else to follow.
I would seriously consider a shorter route – if your training sessions (good idea) have left them battered then that’s probably given you some insight into their hill fitness. I’m a runner as well as hill walker and the fitness – both physical and mental – are quite different. Being good runners does not mean they’ll do well on the hill.
I would do something shorter, see how they enjoy it and build up to the longer route over time. Beasting them for their first big trip in the mountains may well put them off for life…