Bit confused over the advice. DS2 is 3 months and about to move him from his moses basket to a cot in our room. Was going to use same cot and mattress that we had for DS1 (2 years) which was new when we bought it. As it's been used for under 2 years it's in good condition and we've never had any spillages etc on the mattress. Can we use it for DS2 or should I buy an entirely new mattress?