My son getting out of the pushchair

[katanneh]

Hello,
We recently got a second hand Egg tandem pushchair in preparation for our second baby due very soon. We've started using it in single mode and got rid of our old single one.

My 2 year old son has worked out how to press the button, undo the strap and stand up in the pushchair, grinning cheekily and saying "hello mummy, hello daddy!" Realising how clever he's been.

We bought a "Houdini Stop" extra strap which worked for 2 minutes until he realised how to undo that too.

He is so so active that toddler reins wouldn't work, just induce tantrums. He isn't at the stage where he's danger aware enough to have him walk or use a buggy board without running away. We live in a city so these options don't feel safe.

I'm currently using our very tatty travel pushchair which doesn't work for two children but has a stiffer button.

I'm wondering if any brands have especially good clasps that are trickier to undo that anyone could recommend? Or any better products than the Houdini Stop?

I'm really nervous about how I'm going to go out and about with them both when the new baby arrives!

Any ideas?

Thanks in advance.