Well I certainly wouldn't do it!
Can you not just pay for extra legroom and have an exit aisle, with the children sat in front or behind you.
It's not exactly a family holiday if the family are separated.
And no, I wouldn't want someone else's unsupervised child to look after either.
Why not just chose the type of holiday you can afford for the whole family rather than putting some members of the family as being more important than others?