My two older daughters were 11 and 13 when the Baby was born. She was a bonus baby, a huge surprise I was under the impression I couldn't get pregnant anymore (I had health issues, surgeries and had virtually been told there was too much scar tissue etc to have more babies.)
I had my first two 26 months apart and it was a LOT easier to have a newborn with two young teens than with toddlers! A lot of people said things like, "Oh, you now have built in baby sitters." Which my older girls hated to hear (although they loved to play with their baby sister.) But, a LLL friend of mine said, "Oh, the best form of birth control." She was right. My girls learned when they were old enough to appreciate it how difficult a new baby is. That babies are not "your friends" and that they usually take a lot more than they give in the beginning and at least in part, my girls have been very responsible about sexual partners, using birth control consistently.... and I'm not a grandma yet! (And glad not to be. My girls have things to do with their lives before they settle down and have children. In fact, I told them, "Get your Master's and even maybe your PhD before you start popping out babies. It's so hard with small ones." I got my Master's with little ones in the house, and I wouldn't recommend it, (unless it's the only way) as a choice.
Anyway, my girls learned while still young and impressionable the true work that little babies require as well as the rewards. As result neither of the older ones have kids yet, but I'm not anxious, it's their lives and they will have children if and when they are ready. They still learned a LOT about babies, breastfeeding, Attachment Parenting, cloth diapering, gentle discipline, what they call "Hippy Parenting" when they were old enough to appreciate and assimilate these things. I think that's a good thing. My youngest had two more people to love and who loved her, that could not only help out, but whom she could bond with and love. Kind of like aunties, but a bit younger and closer.
In fact, I feel bad that our youngest doesn't have the access to babies that her sisters did.
Congratulations on your pregnancy! It will be quite a ride and very informative for your children as well as good for them. It doesn't hurt that someone besides DP can help with diapers and entertaining the little one, either. My older ones also were very helpful at LLL meetings and with the babies and toddlers of the Lactation clients I used to see in my home. All in all, we didn't plan it, but it was a good thing for everybody.