we use one of the armless sleeping bags even at home (except now in the simmer here in France).
He always used to sleep in the tent in just that, and if it was cold we'd put a duvet jaclet over him (again I'm talking France camping here).
When about 6 months he would sleep in his car bed (lie flat job, not a maxi cosi). Now he's 19 months he sleeps on a normal mat, in a baby gro, and in a kids sleeping bag, which is folded underneath him. The belt trick described above can also help.
When just in the kids sleeping bag he would wriggle out. The belt/folding underneath trick is more for conserving his heat in a sensible s ag area (i.e no pint heating up all the sack if he's not using it as he will get colder (a common trick for small women/ kids too).