The right answers are 2:
1 --> No dejes a los niños ir al parque todavía/aún.
(no dejes a los niños = don't let the kids // ir al parque todavía = go to the park yet)
2 --> No dejes que los niños vayan al parque todavía/aún.
(no dejes que los niños = don't allow the kids // vayan al parque todavía = to go to the park yet)
* todavía = aún (totally the same)
When we ask someone to don't do something, the construction is always
[ No + (2nd person present subjunctive verb) ]
--> No fumes aquí = Don't smoke here.
(fumes -> 2nd person present subjunctive)
--> No me digas que no puedo fumar = Don't tell me that I can't smoke.
(digas -> 2nd person present subjunctive)
So the negative imperative uses present subjunctive, while positive imperative uses its own form.
- Fuma, por favor = (you) smoke, please.
- No fumes, por favor = don't (you) smoke, please.
- Di / Dime, por favor = Tell / Tell me, please.
- No digas / No me digas, por favor = Don't tell / Don't tell me, please.
Well, we can see another subjunctive verb in the example 2, which is "vayan", from the verb "ir" but conjugated in 3rd person plural present tense subjunctive (ellos vayan). This is another construction to learn:
[ No + (verb) + que + (subjunctive verb) ]
--> No quiero que vengas a mi casa = I don't want you to come to my house.
(quiero -> 1st person present indicative of "querer")
(vengas -> 2nd person present subjunctive of "venir")
--> No esperes que yo quiera ir = Don't (you) expect me to want to go.
(esperes -> 2nd person present subjunctive of "esperar")
(quiera -> 1st person present subjunctive of "querer")
I'm not sure if "expect me to want to go" is correct, should it be "expect me want to go"?
As you can see, the uses of subjunctive mode in Spanish are many, and a real pain in the ass xD Even for native Spanish teachers is a nightmare to explain it to people that don't know a thing about it. It is super easy for us when talking because of a whole life using it, you know, but when we have to explain it, it's when we get how hard it is :'0
But fear not. This is a very important part of Spanish language (the half of it maybe) and you can learn it by studying the different rules or grammar structures it uses. Subjunctive is about memorizing them one at a time and practicing it with examples. I'm sure there's a lot of videos on YouTube of English native speakers explaining some of them in a way you can understand better, maybe.