You shouldn't be afraid of them at all. At all stages of the game Sion can't kill Poppy easily, and after a certain point, he can't kill her at all. If you are finding yourself getting poked out by Cho, if you see him walking towards you to Q, walk towards him and e onto him while his Q is still building under you and proceed to trade favorably since his Q is most of his damage really early on. If he starts dropping his Q right in front of him or right on top of himself, juke him out by not using E and watch him waste mana and a cooldown.
As for Sion, Always be walking around him when you are trading with him. There is nothing a Sion hates more than a target that moves perpendicular to him. It makes landing a charged Q a nightmare, and that is a huge chunk of his damage he's missing out on. Remember that you can interupt his Q with your E and, when you choose to do so, you can bruise him up quite a bit, but just remember that his Q will be back in 2 seconds, and to always be walking around him while you are autoing him