If that can't be arranged, you reschedule the call.
You do understand that sometimes the alternative to the kid sitting quietly in their parent's lap is the kid screaming their head off on the other side of the home office door, right?
Personally, I'll take the kid sitting in the parent's lap as being the lesser by far of two annoyances, thanks.
Whoever is looking after the child should not be allowing them to sit outside the office door and scream! If you're home alone with a kid, you shouldn't be taking conference calls (except under emergency situations e.g. sick kid + critical call).
Right, so only people in two-caregiver households get to work from home. Got it.
Everybody up and down this thread who is assuming that there even is another caregiver needs to check their privilege.
I am not a parent myself, but I have been on conference calls where a colleague tried to keep a child out of their home office for a few minutes (like, for the part of the call when it was their turn to give an update) and it blew up in their face. Just let the kid sit in your lap already, it's fine.
edit: If it lends any weight to my opinion, I guess I could also point out I work from home and that that despite your assumption I've been the only parent for two children <= 5 years old for the last 9 weeks.
The need isn't particularly controversial, the availability and affordability is.
I'm not saying is the right or wrong thing to do, I'm just saying the message is clear.