Including adult children from previous marriage

Personally if I had one of my children in the bridal party I would have them all

Yes they all should have a role I think in one way or another otherwise they will feel left out & you don't want that... just be creative with what they can do... may be he shouldn't have any groomsmen & just have his children stand by his side? I'm not 100% sure since it's such different age ranges... as for who pays for what I would think the older kids can handle paying for whatever they need to pay for depending on their financial state the younger ones I would think you guys pay for them??? Or if you guys have the financial means to pay for all of them maybe you do that.... think of it this way if the children were younger the parents always pay so I'm not sure how it is when they are older with their own jobs but maybe it's still proper for the parents to pay?

I don't see how you can include one without the others. And as their parents, adult or not, I think you should cover their attire, assuming that's what you're referring to when you ask who pays. A 13-year old, and probably a 20-year old are not going to have money to buy a dress or suit. The 30-somethings probably can, I would hope they have jobs at their ages, but I'd still pay for them regardless.

All four of ours are part of the party, two of whom are under 18. We are covering all wedding related costs.

I would include them all as I am doing with my 3 kids 2, 7 and 10 . But at 30 if have them pay for their own .. seriously lol you raise children and pay for them their entire lives . . And I'm sure help out frequently .. im 30 .. we buy our own stuff , our kids clothes ect..

I completely agree with you have one you have them all. My question is do they all have to be in the wedding party or can they do other things such as programs, flowers, cake cutting etc.?

They should really all be in the wedding party, IMO. How would they be involved with flowers or cake cutting?

If you are having one child in the bridal party, I'd suggest having them all. That way, you avoid hurt feelings. What costs specifically are you referring to?

We have 8 children (4 and 4) respectively. All are in our wedding and we will cover their attire, HMU, etc. they range in age from 29 to 15. ☺️

Flipping the perspective, I'd be really offended if my mom got married and asked my sister to be her MOH and told me I was a guest "but your name will be in the program." Watching one sibling be honored by my parent like that and knowing I didn't get a similar consideration would hurt. I'd also pay for them all, if you're able to. The 13 year old certainly would need help, and the 20 year old may too. A nice gesture for only half of them wouldn't feel right to me, so I'd offer the same to the oldest two as well.