Ashton Court and Leigh Woods are really easy but fun. No real climb to talk of (unless you go down the steep off piste at Leigh Woods – which you wouldn’t be).
They’ve been surfaced so you could just about balance bike them / kids bikes would be fine, but if you’re good you could clear the rollers at speed.
Not sure about Brechfa, I only did the black / red loop there. I did accidentally stray onto a blue section and it had fun massive berms built into it.
Definitely nothing at Cwmcarn – all big climbs and fast descents.
Not a loop but Flyup 417 at cheltenham could be worth a look – although you’d probably want to pay for uplift as the climb is horrible in the middle (steel and gravelly). There is a very easy but flowy blue designed pretty much for kids (although it’s also fun for adults on a hardtail) – and there is another blue that’s a little bit harder but still very much a flow trail where you can keep your wheels on the ground. There’s also a pumptrack in a barn. Cost is probably prohibitive though for 3 of you uplifting.
Just a sudden thought – what about Cwm Rhayader just north of Brechfa?