Coed y Brenin has a small activty park,swings,slides and small climbing frame,cafe is good.They also have activies for kids during the summer maybe they still do on weekends.
The Afon route is for families and small kids,basically a forest road,balance bike will be ok on it but a bit of an uphill trek back to the centre.
Ffestiniog railway is good but for young kids might be a bit long in my view,forecast is good so stunning scenery.
Can i suggest Portmeirion 20 mins from CYB,kids love it there,shops cafes and very nice walks.