What sort of clips have you tried? These ones from jojo work really well for us on a toddler bed with a standard toddler size duvet. There are 2 clips attached to each other with elastic. The clips attach to each side of the duvet and the elastic goes under the mattress which prevents the duvet sliding off. The clips hold onto the duvet very securely (if you read the instructions - I didn't first time) and they have never pulled off, I tied a couple of knots in the elastic to get a tighter fit (think I got that idea from one of the customer reviews). I also looked at the gro bedding but was concerned about the zips irritating my son if we moved onto using the duvet cover without zipping it to the bottom sheet. Plus jojo clips are only £7. We have attached the clips about a 1/3 old the way up the bed so the duvet can still slip off my toddlers top half but we want to make sure he can get in and out easily. I guess you could buy 2 sets if you want to ensure it didn't slip down at all.
http://www.jojomamanbebe.co.uk/sp+duvet-clips-in-quilts-and-bedding+a3071

Try a single duvet? We realised that our son was a wriggler in bed, so although he still fitted into the bed it didn't take much for his duvet to fall off. The bigger duvet was a bit heavier so didn't move as much.

Same thing happened to us - two solutions I found that work well are either tucking a blanket or sheet (depending on weather) over the top of the duvet round all the sides. Or we have a single sized duvet on the toddler bed widthways so that half the duvet is under the mattress. Works really well as the duvet doesn't fall on the floor and my daughter can still poke her leg out if she gets too hot. Hope that makes sense / helps?!
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I think Vertbaudet do something similar so it's like a giant sleeping bag

Hello, have you considered the Baby Gro integrated duvet? Admittedly its a bit pricey but works a treat for my toddler - basically a zip attaching duvet to the sheet - works really well and my 2 year old very comfy in it- and he is a very committed wriggler

My child has always been a brilliant sleeper but since we removed a railing the duvet keeps falling off and waking her up... ie. this morning at 5am it was fully on the floor. I know she needs to learn to right it herself but if anyone knows any tricks or gadgets to help I would be grateful? I have tried the duvet clip but that was next to useless and the duvet isnt really big enough to tuck on both sides..

Many thanks