If your answer is yes to the above question, they might be sleeping on a mattress that may not be suitable for their body's physical structure. Not every mattress is universal enough to cater to the different needs of everyone. Each and every one of our kids is unique, and this requires us to take note that it is a 'case to case' basis when we purchase something to support them in their sleep.
I would recommend you to go online to browse through the many available mattresses, and find out more on their varied specifications (one that can fit into most of your requirements). If you feel confident that you have found the right one, you can actually purchase it with just a few clicks of your mouse, in the comfort of your own home! If not, it'd be a good idea too, to head down to a real showroom to try lying down on one. As mentioned, there may be a wide variety of products to choose from, and keep the following guidelines in mind, can help you narrow down your choices even quicker:
Seek a detailed explanation on the physical aspects of a particular mattress
You should know by now that there is no "one style fits all". One of the few components, you need to pay attention to are the coils and the springs that are within a mattress. They are the ones that are responsible in providing you the optimal support that you need. The number of the mentioned components varies between different mattresses. Another general rule to note is the mattress's depth and padding type that's also located within – I'd recommend you start looking from at least 7 inches to 16 inches or more.
Look for a mattress that specifically mentions having "back-support" capabilities
Often, companies will state whether their products now have back support technicalities, like the ability to memorize a user's unique curve or arch when he/she uses it long enough. If it isn't stated, you can also look for mattress types that have manufacturer's notes stating that it is a medium-firm. This way, your kids will not "sink in" too much into the bed (it means that the mattress is, in other words, weak to support them). Choosing this type of specialized mattresses can reduce your morning muscle soreness to a minimal.
Ultimately, listen to your kids. They know best on what they want or need
It's recommended that you are open to any external advice from your kids, and actually head down to an available try-it-out outlet to let them lie down on a mattress that you are interested in buying beforehand. Only after they have tried it out, you will be able to better decide if it's the one you're looking for.
That is where your kids can help your decision making, a mattress that you deemed as "the one" the other day, may not be the one you'll buy on the actual day. So I would suggest that you take your time to decide, and not rush into any rash decisions that you might regret later on.