Can't express why I'm so angry

[EmmaAsking]

We've got an allotment next to a fairly busy road. There's no direct access to the road, it's about a 200m walk down two paths.
My partner was meant to be looking after our 2 year old while I went home to get a coat. When I came back about 15 minutes later our toddler was at the side of the road, on his own. He'd left the allotment, and apparently my partner thought he'd stay nearby so didn't follow him. My partner didn't know I was with him. He didn't show up at the road side for maybe three minutes, didn't seem upset, didn't apologise. He said later that he'd been really worried and just hadn't shown it. I'm feeling furious and disconnected and just generally full of dread and he doesn't seem to get why. I kinda think it's obvious but that's not getting us anywhere. Am I overreacting? Any advice?

I know EXACTLY how this feels. I don't know if most men are like this, or if mothers are generally more alert to danger tbh, but it feels awful when you are worried about something and they just don't care.

When I was 17 I was walking to my local stables and literally watched a toddler, maybe 18 months old walking from the pavement on to the road between parked cars. I panicked, ran to grab the kid and then stood on the pavement with this child for minutes before anyone arrived. Some mum came out of a house and said 'oh there you are', took the kid from me and went back inside. It was 28 years ago and I've never forgotten. My mum was shocked when I told her. Before phones etc. I still shudder when I think of an alternative scenario there.
I'm shocked on your behalf.

You need eyes in the back of your head when they are that age, it's a 24/7 occupation

But he doesn't seem to have realized that yet