I grew up in foster care and my grandparents who raised me for most of my life before then are both deceased, so I understand what it’s like to not have close family ties.
Teach your kids that family isn’t always about blood. Sometimes family is the people you choose as family. If you don’t have any friends you’d count as family, then work on that. Not just for your kids, but for you too.
If you do have any family at all (assuming there is no negative pat history involved, I don’t know your full situation), consider reaching out more. It can be awkward at first when the family dynamics are unfamiliar, but it’s worth a shot. You may have to keep reaching out and continue trying.
Does your partner (assuming you have one— I read family of 4 but didn’t see a specific reference to a partner so I wasn’t sure if it was 2 partners and 2 kids or you and 3 kids, sorry!) have any family either? That’s always a possible source.
Beyond that, get involved in groups, activities, kid’s school/extracurricular activities. Get out there and start meeting some people! You might not have been given the family you want, but you can choose to make a family of your own design.
And if you can’t? That’s okay too. Remember that social media is very deceptive. Of course you see all the parties, events, get togethers with big happy families. Nobody gets on social media and blasts “I’m so lonely, my family sucks, we have nobody,” do they? You just see all the good parts. So don’t beat yourself up about it and your kids will most certainly turn out fine. Nowhere in my psych undergrad courses did we cover where kids turn out maladjusted because they didn’t have a big extended family. 🙃 Your kids are fine, so just put yourself out there to see who you meet and don’t put pressure on yourself if it doesn’t work out.