I think even the EN13537 ratings are likely to leave you feeling a bit cold, especially if any of your kids are female, and a no-name brand bag is unlikely to have an EN rating as the test setup is expensive.
The most useful rule of thumb is loft comparisons. Western Mountaineering and Katabatic Gear are known to be a bit conservative in their temperature ratings, and use the same rules of thumb. You can see all of WM's (double-sided) loft and temperature specs here:
http://www.westernmountaineering.com/sl ... ion-chart/BPL also have a rule of thumb, which runs a little colder than WM's specs:
https://backpackinglight.com/bpl_sleepi ... statement/If you find a bag with an EN rating and want to know what that correlates to in loft (single-sided, top layer), Richard Nisley posted a great chart in the link below that allows you to derive the loft of the top layer (assuming no funky fill distributions):
https://backpackinglight.com/forums/top ... /#commentsIf you cannot find an EN rating or a loft figure, and cannot measure the loft for yourself, you're buying on trust and I haven't seen very many other products where less reputable brands have such egregious false advertising. It's usually cheaper to buy once and buy right than buy twice.
Don't let sales people sell you on supposed fill power numbers. Higher fill power only decreases the weight required to achieve a certain loft, but loft is what keeps you warm. Also, fill power is another thing that they often fib about.
Synthetic bags are a bit different to the above. They seem to require less loft for a given temp reading. Again, there's good discussion on BPL regarding this.