The idea itself isn't bad persay but there is definitely good and bad aspects to it. It would be easier to change water on 1 system than 2,you don't need a second skimmer and it adds more water volume. Now the bad..if any of those fish get a disease or ich let's say then they are all going to get it. My main concern though would be the kid(s) How old is your daughter? How do you plan to keep the sump and plumbing protected? Against contamination, against kids playing, wrestling,kicking balls,throwing stuff playing with pumps in sump, taking out rocks,heaters etc. Heaters could definitely break from not being submerged,burn a child or other objects it was on if dry. Trust me heaters get very hot I have burned myself once or twice thinking it was off and cool. Just breaking down all the different ideas I can think of good and bad. In the end you really just want the safety of your child and your tank.
On a side note two SEPARATE systems is NOT a bad idea. Yes maybe some more electric and another skimmer/filter. BUT if something was to happen to 1 system like if the heater went bad making it cold or creating stray current, something got into it messing up your parameters, and numerous other options that could happen you can always just put your livestock into the other system. One of our vendors here for example had his system all on 1 pump, plumbed together and someone poisoned it and they lost everything. Had they been separate tanks it would have only killed 1 tank.
It really all comes down to what you want to do. Weigh out the options and take them all into consideration.
