I see. Again this is very odd.
What you need to find out is what reason the local authority gave for not returning your children to your care.
You need to know this as if you are going to apply to court for contact and/ or to get them home, you have to show that whatever they were saying about you back in 2017 no longer applies.
This is what you will have to work with - even if you don't accept that what was said at the time was true. It's been 4 years now and as such you should be able to show that you are a different person from the person they say you were back then.
Are you wanting them to come home or are you just looking for contact to see them at this moment in time? It might be better to start by applying for contact before making an application to overturn the court order, as if you can establish a relationship with them again that is a big positive.
You would need to make a c100 application for contact, filing an additional c15 form as they are in care. The forms are here:
The cost to apply is £215, but you can also possibly get a fee exemption if you are on low income/have low savings.
For this you would need to fill out a fee exemption form: http://www.gov.uk/get-help-with-court-fees
I hope that this helps you today?