I already have ILR. Do I need to apply to the EU Settlement Scheme for settled status?

If you have proof of your ILR (Indefinite Leave to Remain), you do not need to apply to the EU Settlement Scheme.

However, you will not be covered by the Withdrawal Agreement if you do not apply, which means you will lose your ILR after a 2-year absence from the UK. People with Settled Status on the other hand only lose their status after a 5-year absence from the UK. There are some other differences such as family reunion rights under the Withdrawal Agreement.

If you came to the UK before 1989, but you do not have ILR or have lost your evidence of ILR, then you and your children can choose to apply to the Windrush Scheme to get ILR - if you do not want to apply to the EU Settlement Scheme.

This page https://www.the3million.org.uk/windrush shows more details, and also contains a table of the difference in rights between ILR and Settled Status.