When you marry your husband you can use your marriage certificate as evidence to change your surname.
As far as your kids go, you will need to apply for their surnames to be officially changed.
Unless any of your children are over 18 you need to apply to have any part of their name officially changed. You may be able to use the new surname "in common" (ie just start using it) but without being able to provide proof of the change on their behalf all official documents (incl. school records etc) will be in their current surname.
To change a child's surname you need to fill in a form at Births, deaths and marriages in your state with the written approval from both parents. Unless your fiance has been granted parental responsibility in a legal sense either through gaurdianship or adoption you will need your previous partner to sign the form to approve the change for your son from a previous relationship.
Hope that helps
My daughter from a previous relationship has always had my surname. When i got married though my fiance changed his surname instead, using the marriage certificate as proof, rather than us needing to change both mine and my daughters surnames. Turned out the easier option for us