If you are checking bags then you can put what sunscreen you like in your checked luggage because the 100ml limit only applies to hand baggage. If you are using hand baggage only then whilst in transit in DUbai you will have access to all the shopping in DXB and I am sure that they sell sunscreen at the chemist outlets so you should be fine to buy them there in any quantity you like because I do not believe that they rescreen you before you board (they do rescreen US bound flights at the gate but that is the only route that I know that they do this for).