Perfect time for a little sex education. We did it with our kids.
"Mommy and dad have decided our family is big enough. We decided daddy will get a vasectomy to stop babies. He will have a little surgery and will need a little TLC afterwards " We gave rules on how to snuggle with daddy for a few days.
By the time my dh had his vasectomy the older two kids had an idea about how babies were made. It took a part from mommy and a part from daddy. It took 2 more years for my son to finally ask how daddy's part get into mommy's part. My oldest dd just "figured it out" on her own. We still discussed it.
Then let the conversation go from there. Vasectomy, vas deferense, penis, testicles, sperm, egg, womb/uterus, vagina are not bad nor are they dirty words. Sex education should be an on going conversation.