Hello! I haven't been on here for a long while, but I'm having a bit of anxiety about this and wanted to have some rational answers.
The situation: Without going into too much detail, DH, toddler (18m) and I are staying with in-laws for a while, toddler is sleeping in a travel cot. We're planning a family trip, but I'll be traveling to destination by car (taking the cot with me) in the evening, and DH and toddler will travel to destination by train the next day. This means that, for one night, toddler won't be able to sleep in her cot.
The toddler: She's used to always sleeping in a cot, uses a sleep bag and is fine, sleeps through (finally), etc. When we cover her with a baby blanket, it mostly stays on her, so I suppose she's not that much of a wriggler. She's large (95 percentile), mobile, walks well, very strong (e.g. it's starting to be really difficult to put her shoes on if she's not willing, will jump out of my arms, etc).
The issue/question: DH is planning on bed-sharing a single bed against the wall for that night, with her between the wall and himself. We have both co-slept before on occasion in a single bed in her room (against a wall), mostly when she wakes up too early or is unsettled, but of course this means we're not fully asleep, and neither is she. She has never slept in our bed, and I don't think she'd care for it. DH is a very involved parent, rather slim, not a heavy sleeper, doesn't drink, smoke, used to looking after her through the night, etc, so no issues there.
My concern is that she will suffocate. That she will somehow fall between mattress and wall (for reference, she's in 18-24m clothes and 12.5kg), or that she will get trapped under the adult-size duvet or plaster her face on her dad's pillow in her sleep. To prevent her from falling off the single bed at night, his plan is to tuck her in tightly while he's still awake (having dinner, watching tv, etc), then get into bed with her and serve as a guard-rail himself. He'll also place a mattress/rug next to the bed in case she still wriggles out and falls.
How realistic are my fears here? I'm super risk-averse, but DH is really not, so sometimes it's a struggle for me to have a balanced perspective. My rational mind tells me that I have a 1.5 child who is big, strong, and fully mobile, and therefore she would be able to instinctively get out of any pesky situation by herself and wouldn't suffocate in her sleep. It also tells me that families have been sharing beds (big or small) for centuries, and most children didn't die from this. My anxiety tells me that all recommendations are for infants and young toddlers not to sleep near adult-sized duvets or pillows (but surely most co-sleeping parents of older infants don't follow this to the T, especially in long-term arrangements and cold climates?).