Most 2 year olds are very safe sleeping with a blanket however yes it's possible that they could suffocate under a blanket but very unlikely.
At 2 years old they are usually able to get there mouths and face away from the blanket if they begin to have trouble breathing.
A 2 year old should also sleep in a crib until they are around 3 years old and then you can move the child to a toddler bed.
The crib will keep the 2 year old from leaving their bed and possibly rolling out of their bed and getting injured.
I would recommend using a light blanket that is very breathable to prevent the possibility of suffocation.
However if you want to keep your 2 year old warm enough without a blanket you can try putting the 2 year old in some blanket sleepers or footed pajamas to keep them warm.
But at even 1 year old you can begin using a light blanket in the toddlers crib.