I used to use completely sugar free but now I've eased up a bit. My son (2.5 years) regularly has a choice of:
Ready Brek (completely sugar free)
Weetabix (pretty low sugar)
Shreddies (pretty low sugar)
Rice Krispies (pretty low sugar)
Cheerios (a bit higher in sugar)
Home made bircher muesli (natural sugars only)
What I'd steer clear of is the chocolate ones and the sugar 'frosted' ones. Basically that's just like eating sweets. That said, my son does still regularly have wholemeal toast with honey on it but to me that seems better than something excessively artificial. It's also stopped me and my husband gorging ourselves on Cinnamon Grahams as we used to because our son just wants to eat what we have.