I suggest you go the other way.
Any place you pick that’s open to the public risks other people showing up. I’d look for a private place where the owners will assure you privacy.
If privacy is your goal, that’s what I’d do.
Sorry if off topic.
Can the kid ride a bike yet? Kick a football? Swim?
I love reading now, but when I was 5 I only wanted to look at the pictures in books, not the words.
My friend hates to fishing, because he dad tried to force it on him before he was ready.
And if he is interested, it’s probably better you build one together than buy one.
imho.
Enjoy!