I didn't vote, as I think it's very much a case-by-case situation.
For example, a young baby who mostly sleeps is very different from an alert, intelligent toddler. (I'd be ok with my friend bringing a baby, but might ask to reschedule if she's bringing her toddler).
What is the activity you're doing, and what time of day are you meeting? A coffee shop at 10am? I'd gladly tell my friend to bring her child. A wine bar at 8pm? I'd ask to reschedule.
What are my friend's circumstances? Is she a single mum, or someone who doesn't have access to a trusted and reliable babysitter? I'd cut her some slack and encourage her to bring her child. Similarly, if she works full-time, and has very little spare time to spend with her child, I wouldn't quibble if she wanted to bring them along.
Is this just a casual catch-up, or do I have big / sad / scary news to impart to her? I'd go ahead in the former case, but ask to reschedule for the latter.
Finally OP, I've often been the childless friend in this situation. I find a great solution is to go around to my friend's place, rather than us going 'out'. The kid(s) are there, and I interact with them for a bit, but they invariably drift away to play, or have a nap, etc, and my friend and I get to catch up. Win-win for everyone.