Zips, buttons, cords

Hi. I'm finding the 3-4 age trousers seem to have cords to tie them up around the waist and some even have (mainly jeans) zips and buttons. Are our 3 year olds supposed to be able to do buttons and tie up cords at this age!? I've even heard of horror stories where little boys have got their penis stuck in zips resulting in hospital visits. I've only put the zip/button trousers on on the days he's with me all day. nursery days have to be the corded ones cos he won't ask them to tie it up. Yesterday when we were walking back to the car his trousers came down cos he hadn't got them tied by the nursery. We've only just moved over to 3-4 trousers cos his bottom area is smaller. The 2-3 fit around the waist still but leg length is short. I know makes like Next have a button tie elastic inside but can't afford to buy all Next clothes. I'm a pre-owned/ebay/Primark shopper. I preferred the younger age trousers where everything was elastic only. Any advice and can yours do buttons and tieing??