Yes, this is the best course of action. A separate sleeping place is the safest way to go. Unsafe sleep environments are a major factor in sleep-related deaths, and you are quite correct in pointing that out. No matter how cautious one thinks they are.
While the peak risk for SIDS has passed by 16 months, an adult mattress still isn’t designed to be **a safe sleep space** for a toddler, **regardless of firmness.** Adult beds introduce hazards like gaps, soft surfaces, pillows, and entrapment risks that simply aren’t present in a properly designed crib or toddler bed. So even though the danger isn’t the same as it is for an infant, it’s still significantly safer for a child this age to sleep in their own designated sleep space. Quite frankly, I am not sure why there is this obsession to coshare in the first place, but that is just my opinion.
It’s also true that most cases of sleep-related deaths in the U.S., including those classified as SIDS, occur with at least one unsafe sleep factor present. That’s why major pediatric organizations continue to caution that bedsharing remains risky beyond infancy, even though the highest danger is in the first year. It is not just the firmness issue. The safest option is always a separate, firm, uncluttered sleep surface, and encouraging that choice helps keep kids as safe as possible while acknowledging and coinciding with what the data actually shows.
Your advice to keep kids in their own safe sleeping environment is the best practice.
Most folks like to go with quality latex, in a firmer feel. I got my grandson a birch kids as he is a toddler sleeping in a floor bed, and my grand daughter and avocado crib mattress both with avocado's mattress protector which uses cornstarch to create the water proof layer between the organic cotton top and bottom layer of the protector, but still very breathable.
So any Avocado or Birch, firmer mattress will be suitable for what you are looking for, as will a MyGreenMattress, SleepEZ, Arizona Premium or naturepedic, when it comes to being hypoallergenic.
There are some certipur non latex mattresses, latex is likey your best bet here for what you describe and for piece of mind.
The best practice is to do this with separate sleeping spaces.
Actually, an adult mattress labeled “firm” can often be much firmer than a typical kids’ or toddler mattress, though not always as firm as an infant crib mattress. It really comes down to what you mean by firm and how that firmness changes depending on the sleeper’s weight. For adults, a firm feel usually falls around 8–8.5/10 and up on standard firmness scales.
For infants, you want a completely flat, very firm, low-flex mattress because their motor control and airway protection abilities are still developing. Toddlers and young children, however, generally sleep best on something in the medium-firm to slightly-firm range to support proper spinal alignment as they grow.
Adult mattress “firm” labels can be misleading, since some so-called firm models include pillow-tops, Euro-tops, or plush comfort layers that are far too soft for a toddler. But a true latex firm—especially one built with a firmer support core and comfort layers in the 38 ILD to extra-firm 44 ILD range, can provide adequate support for a toddler and older child. (Though 44 ILD may be unnecessarily firm for most kids.)
The key distinction is that crib mattresses need to be very firm with almost no flex, while toddlers transition to a different level of firmness that supports alignment without the absolute rigidity required for infants.
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