Hello! I bought the Smart Bottoms Mattress Bad to place under my LO when bedsharing. We were having major pee leaks every night since we were struggling to find a cloth diaper option that worked. We now have two of these mattress pads, and have been using them since babe was about 3 months old (we used nothing up until that point).
Pros:
-thick enough that it won't come up and risk covering babe, although it does bunch a little. I feel it is safe as long as I am lying on half of it holding it down, as it would only be moving between us when she's rolling in to nurse if it were to move. So baby is lying in the middle or slightly to one side, and I'm right up next to her in the mama cuddle curl holding down the other side of the mat.
-soft and comfy for sleeping on
-definitely waterproof!
-washes up very well... we wash our mattress pad almost every day, and it so far shows no sign of wear at all.
-good size - not too big, not too small.
-triples as a mat for naked baby playtime and as a diaper change pad for on the go.
Cons:
-bunches up a little, you might feel you need to flatten it back down during the night (this is no big deal, I barely even need to wake up to do this, plus the bunches aren't significant enough to be a danger to her, IMO).
Obviously if buying this for bedsharing you want to be careful when using (I believe you aren't really supposed to put anything underneath, according to the official rules). But at no time have I felt this was any kind of hazard to my little girl while she slept, and I've been using these mats for four months now, every night. Maybe for really small babies it would be more of a concern, but my 6 month old got really big and strong really fast and has no health concerns, so if there is an increased risk it is very minuscule (she has weighed as much as a baby almost twice her age basically since around 3 months old, and she rolled when she turned 4 months). Hope that helps!