Hi ladies. We are buying our first cotbed mattress for little one who will soon go in his crib. We got one given but I'm not comfortable reusing one as I believe they advise to have a new one for each kid. Any tips on what to look out for when mattress buying? Any ideas on safety, comfort, practicalities welcome! It will last us many years as it's for a cotbed so want to make sure we get it right.