Went last year to Sant Tomas, it was lovely but very, very quiet - don't know what Cala Blanca is like, sorry. We hired a car, so again, no info on buses.
We went with DD who was 3.6 at the time and she loved going to Cala Galdana, it's a lovely beach and there is a fab cafe on the front (look for the huge red tap suspended in midair) with waterslides, she would have stayed there all day if we'd let her!
we did a beach-hopping boat trip from Citadella that was suitable for non-swimmers - they anchored up in coves by rocks and you were able to walk to the beach - that was a nice day out.
Mahon (Mao) itself is lovely and well worth a nose around.
Fornells is also nice - big marina with some nice restaurants.
Wouldn't recommend the big mountain with the cathedral on top with children - stiflingly hot and although the views are good it's basically a church with a massive cafe/gift shop attached and nothing else - a long drive for very little!
We also went to the horse show - I'm a horse addict and I loved it, DD liked it a bit (loved shouting Ole! when the horses did their jumping) but it does drag on a bit and if you're not into horses (like DH) you may well be bored rigid.
If there's a fiesta on (they seem to have lots and lots of them!) then you'd get to see the horses in action for free...
Anyway, DD spent most of the holiday glued to the hotel pool so I'm sorry I can't be any more use!