Here’s a quick poll - what would be your response to the following situation:
Approaching your home, which is on a fairly busy road leading to a school, you see a woman with a preschooler and a toddler walking in front of you. As you near your gate, the toddler darts inside and sits down on the lawn. The woman says “come out of that garden”, and lifts him out over the fence. What do you do?
I wouldn’t care if she even had to enter the garden herself to get the toddler out, or if she had to chase him through my petunias. Stuff happens. I’d just make a friendly comment to let the mom know I was understanding, and perhaps small talk.
On a fairly busy road? Smile and make small talk, and be glad the toddler darted into the yard rather than into the street.
I would threaten to call the police and then immediately make an appointment to see an attorney - in the event it ever happened again. I would then be prepared to sue them for every thing they owned, make sure when the kid grows up he could not afford to attend college.
I had a somewhat-similar situation a couple of years ago - a mother walking with her young child allowed the child to run through my flower bed and ruin one of my large flowers. I didn’t say anything, but I was NOT impressed. I don’t see any reason for anyone, including toddlers, to be in my yard.
Is the mom hot?
Get off my lawn (and garden)!
Option 3; say nothing, but get better gate lock (or an ED-209)
“You have illegally entered private property, leave now, you have twenty seconds to comply…”
How do you respond to a small child briefly in your front yard?
I’d steal a line from the T-1000 in Terminator 2:
“Get. Out.”
Brief, yet effective.
You make me LOL 
(<-see?)
Well, I’d like to think so. But I did turn forty just a couple of weeks ago, so YMMV 
Cat Whisperer. No fear of that here. I said “lawn” by courtesy, but a more accurate description would be “weed patch with a tree growing in the middle of it.” I can understand a person being PO’d if something was actually damaged.
And Option 5 was indeed the response of the lady in the house on the way back from school this morning. After I’d already told him to come out and physically removed him. Which kind of freaked me out. I would say it’s been a fair while since someone’s offered to call the police on me!
It’s just a lawn; some grass, some bugs, looks greenish. Who cares?
Shoot first. Ask questions later.
Children need to learn respect for other people’s property. Remove the kid the first time it happens. If it happens a second time? Kill the kid. With fire.
Smile. I had once a beautiful garden and the elementary school teachers used to walk the students by my house. Kids love nature.
The person who paid for the lawn and spends a lot of time, money, and energy looking after it (and who will get fined from the city if they DON’T spend a lot of time, money, and energy looking after it).
Aspidistra, I assume from your response that this is your toddler that you’re allowing onto other people’s yards - I’d put a stop to that, if I were you. Not everyone wants other people’s children on their property, for various reasons (for one, I use pesticide/herbicide on my lawn whenever I feel like it - I have no kids or animals that go in the front yard). You say he didn’t damage anything, but that isn’t the point; the bottom line is, keep your kids off other people’s property. He has no business going in there.
It would depend on whether I had a fence around my yard or not. I might be peeved if they had actually opened the gate and came in. Mostly I’d be surprised because I live out in the country…
I wish you could have multiple options. Because I’d smile and make small talk and make a mental note to have a better lock on my garden. I love my garden and am not really interested in your kid’s antics.
I wasn’t telling the OP to disregard it, I was simply answering the question of what I would do if it was my lawn. My lawn is some grass, some bugs, a couple bushes and a tree. I don’t really care about it.
Personally, I wouldn’t care. In fact, it happens all the time at my house. People playing Frisbee with their kids on the street, kid runs up on the lawn to retrieve the Frisbee, no problems.
That said, it’s your job to control where your toddler goes. If you can keep him from going into traffic, you can keep him off someone’s yard, it’s the same skill involved. What if this woman has a dangerous dog? I don’t feel she reacted very nicely, but it’s her yard, she doesn’t owe you an explanation for her reaction.
If it was my yard as it exists now, I’d shrug it off, let the mom know, “hey, I’ve been there” and, if the child was at all reluctant to leave say, “You know, some people put poison on their grass, honey. I don’t, but you never know what yard is safe. Don’t go in other people’s yards, okay?” (Yeah, I have no problem reinforcing a parent’s own teaching, and since you’re removing him from the yard, I think we’re in agreement that yards are generally private.)
If it was my dream yard, I’d invite you all in to look at my neat-o garden, and probably have the kiddo making herbal bug spray by lunchtime!
Well, no, you were dismissively asking who cares about a lawn. I helpfully pointed out to you who cares about a lawn. 