You might want to discuss this with them, clearly stating the problem, and asking them for ideas on how to resolve it. When people are consulted and help reach a solution, they feel more inclined to cooperate.
Make rules that you can stick with. With the situation you described, it sounds to me like the rule that all toys have to be put away before any snack or meal time might be your best. If they whine they are too hungry, be compassionate, but firm. Get in and help, make a game of it, who can pick up the fastest, who can pick up the most, let's see if we can get it all picked up in 3 min. etc. Giving reminders is always a good idea. In a loving, gentle tone, "Snack time is in 10 min. and toys are put away before snacks." Then 5 min later, another reminder, maybe setting a timer, "When the timer rings in 5 min. we'll pick up for snack time." Then when the timer rings, "Let's see who can pick up the most while I count to ten." Help them to be successful, expect that they will want to. Eventually you will be able to say, "5 min before snacks, what is the rule?" or "In 5 min we'll have snacks, remember the rule." It might happen that someone or all will miss snacks a few times because they did not pick up. That will be very hard to handle, but stick to it. Schedule snack time so that it is before they are really really hungry...a hungry child has a harder time thinking and responding.
When it comes to clothing, why not have the rule that they need to be put away as soon as they are removed, before going to the next event, whatever it is? Of course the rules we make for the children should be rules of the house that everyone follows. Persistence and consistence is really important in teaching new habits, and always from a loving place. These rules are made so that everyone can enjoy their toys and home. When things are kept organized and orderly, it makes living easier and more fun.
Hope some of this is helpful. Blessings to you.