Double barrel name..

[Roozy123]

I have 2 kids already that have their dad's surname.
I am pregnant with my 3rd, by the same man.

I've been married and divorced and don't want to get married again.

I'm coming to the end of this pregnancy and I have this over whelminh feeling about giving him the dad's last name.
I feel like from the start with my other 2 I should have either given them my 2nd name or atleast had it within their name.

My question is, obviously with this 3rd baby he is not registered yet so I could easily double up the last name being his dad's and mine.
My other kids are 3 and 5, obviously have their birth cerft. Etc..... is it far too late to do it to theirs? Is there any way I can add my secound name. Either before their dad's or after?

Any information on this will be much appreciated.
Oh and btw, their dad is 100 percent fine with this he just also doesn't know where I would Even start.

Thanks.

I apologise in advance for my terrible spelling and grammar

So when I had my son dp was in a bad place and not a very nice person, he forced me to give ds his name only. I wanted to double barrel both our names.
He did councilling and seen how wrong he was. We where not able to change the birth certificate as that is a legal document but we hypinated his name using deed poll. This can only be done if the father agrees and signs the paperwork.
Our son now has both our names and we have just registered our dd last week who has both our names on her birth certificate.
I would definalty hypinate dc3

Thank you PleaseGiveMeAShake for your reply.

Have the children got your name as the last name or is it yours, hyphen then his?

I just remembered my sister in law changed her son's last name to my brothers when they got married (She has a son from a previous relationship) so I can also ask for advice from her.

Doing it through deed poll, will that them legally bè their name ?

So on my dd's birth certificate she is (example names) 'lucy myname-partnersname'
Our son was 'josh partnersname' he is now 'Josh myname-partnersname' through deed poll.

Yes deed poll make this their legal name. We have just registered ds with school and they have his new hypinated name as his legal name.

The only down side is you pay for deed poll. If I remember correctly it was £25 about 2 years ago.