We are going to be in the same position very soon! Dd is the same as your ds about 12kg and in size 2 and the same size sleeping bag but she is busting out of it and every morning she is now out of it as she stretches out and pops the buttons!! Anyway it has been really hot here and there have been times that her sleeping bag has been in the washer or whatever so we haven't been able to use it...so what I do is lie her down as if I was going to put her in the sleeping bag And wrap her legs loosely in a sheet...she doesn't seem to notice the difference but she wil do the same as you ds if I put her to bed without it! She is going into a big bed in about a months so I figure I will do the same then and that way if she needs to get up she wont be stuck in her bag to do it....good luck!!!