Motherly Struggle

I have been blessed with pretty calm children. For the most part they listen to me too and are fairly well behaved.
Lately though when I take Isaac anywhere he is doing really well playing. I am a mother that watches them as much as possible but I don't overly hover. But they are young enough that I need to watch them.
I watch them as best as any mother can with having more then 1 child. Isaac is doing well for while and then I look down at Jackson for 5 seconds maybe and look up and Isaac is GONE.
He will just vanish he is small and fast. And even if he was wearing a bright colored shirt I don't see him. One time I didn't even have Jackson with me and still he somehow got by me (I wasn't even looking down at my phone)
My heart drops as I try to find him as fast as possible and pray that in that short time nothing happened to him.
Once I find him its a huge sigh of relief and I try to explain to him in different ways each time that its not okay to leave me without telling me your leaving especially if its to another room or the other side of a huge crowded area.
Every time he cries when I try to explain rights and wrongs of the situation. And I can only do it so well because while doing it i'm also holding on to Jackson so he wont squirm away and vanish too.
He's just at an age that he still doesn't fully grasp that if he leaves he might not find me. Because I've always found him before he realized he doesn't know where I am. He never gets that scared feeling, maybe he needs to feel that to know and understand.
He leaves because he knows there is something better somewhere else or because he just wants to follow "big kids" or he is just done.
It's one of my big nightmares of losing my children, what mother doesn't have that nightmare. There are several living it! And my heart breaks for them.
I will sometimes have moments of how could you lose your children and not notice or whatever. But I watch my children fairly closing and still some how they escape and it reminds me that you don't know anyone's story or situation.
If anything happened I'm sure I would blame myself and go through that whole grieving process but right now I know i'm doing the best I can do for now. And I just have to keep having talks with him until hopefully it sinks in.
I would love more children but every time something like this happens I just think how much harder it would be with more to keep track of them. I mean hopefully Isaac would be better by then and Jackson too but its in my thoughts when deciding the future that's for sure.

It's not fun feeling that way, so scared, worried, and clueless of where your children may have vanished too. And I debt not taking them places without 2 adults but I can't live my life in fear of that everyday and never do things. Him and I will just have to learn what works for us.

I pray every day or night that I can do my best to keep them safe.
If you have advice you can share not that all will work but doesn't hurt to see. Isaac is 2 1/2 and Jackson is 14 months just to realize how much they can understand.
Please be kind too because like I said you may not know how well I keep track of them or don't.