Red flag 1 - your DP's father is a convicted paedophile
Red flag 2 - your DP and his family are still in contact with said convicted paedophile
Red flag 3 - your DP and his family didn't tell you and when they did tell you, they minimised the seriousness of the the convicted paedophile's offences. Before they told you they put you at risk. When they minimised his crimes, they put you at risk.
Red flag 4 - now your DP is an adult he hasn't gone NC.
Red flag 5 - your DP being in contact with relatives who have contact with said convicted paedophile and minimise his offences puts him at risk of being pressured into minimising himself.
Red flag 6 - your DP already wants to invite the convicted paedophile to your wedding. With children.
Your DP says now that any DC you have will not have contact with the convicted paedophile. But that's easy to say, before the DC are born. He will be under pressure from family. As others have said 'meet him one time' 'just one photo' and before you know it, they have contact. You will be vulnerable after the birth, exhausted and may not be up to preventing it. Or worse, you may be ill in hospital and unable to prevent it.
Paedophiles are manipulative. Your presence won't necessarily prevent him offending. Do you trust your DP not to leave them alone with the paedophile? Not to just 'pop to the loo' 'pop to the kitchen to make a cuppa'?
When your children are older, they will see the paedophile as 'grandad' and trust him. How do you prevent them going to visit without you when they're out and about, after school, at weekends?
Let's say the DC don't have contact with the paedophile at all but do have contact with other relatives in contact with the paedophile. They will grow up trusting grandma/auntie/uncle and when they bump into grandma in the park with their friends - they trust grandma so how do you stop them going back to hers to meet the paedophile?
Even if they don't have contact with the paedophile and never do and DP and his other family, by some miracle, don't introduce them to the paedophole - does their behaviour to date suggest you can trust them to make sensible safeguarding choices when looking after your DC? Do you trust them to recognise when someone else's behaviour is questionable?
Let's say you split up. DP is then under even more pressure to let the DC see the paedophile. Your ability to safeguard them is compromised - the threshold to prevent a parent seeing the children or only having supervised contact is very high. Many an abusive father has got unsupervised custody of their child. Once he has unsupervised contact it will be harder to keep the DC away from the paedophile.
Similarly, if, heaven forbid, something happened to you and you were no longer around. Do you trust your grieving DP to resist his family's call for contact?
There is also a separate risk that if you and DP stay together and allow contact, even if DP does it behind your back, that you lose the DC altogether for failing to safeguard them.
Also, people will find out what he did. If you have contact with the paedophile - do you think other parents will let you have their children over to play with your DC? Will you feel safe knowing everyone knows you are in contact with a paedophile?
If your children are abused, will they forgive you for exposing them to risk, knowing that they were in the company of a paedophile? Same question if they're not abused.
I have relatives who were abused as children. They are in their 70s and 80s now. The trauma doesn't go away. One has to live with the knowledge her maternal grandfather raped her at age 9. That when she told her parents, neither believed her.
There are so many risks here. You are so young, so much time to run as far as you can and meet a lovely man to be a father to your children.