Children, under the age of eleven, have unstable magical cores. This is dues to the fact that they have not reached their first magical majority, and thus, the situations they experience shape their core.
Magic is like a muscle. The more you use it, the stronger it becomes. Example; a child raised with everything it could possibly need, with no reason to feel the extreme emotions that trigger accidental magic are likely to be magically weaker than those who are raised in harsh or even abusive environments who frequently feel extreme emotions.
As their core's are unstable, children also have more effects from rituals that influence the body, mind or magic as the do not have set limits yet, which is why a child could accidentally apparate, with no drain at all, and after their eleventh birthday, could feel dead on their feet after performing a simple levitation charm.
Muggleborn children, are, like other magical children, are born with a core. This is assumed that they are the by-product of a rape-and-obliviate, which, despite being illegal, is still unfortunately very common. Or the child is descended from a squib line that has slowly absorbed enough magic through each generation to be considered a wizard.
Squibs, however, are produced due to inbreeding. Where muggles would develop deformities, a squib is born without deformities only due to the fact that the child's magic in the womb has worked to prevent them, and as such, magically exhausted a child's magic, which, when in a state of flux, is dangerous as because it is unlikely to return, which is why there is the possibility that while your first born is a wizard, your second could be a squib.
The by-product of a wizard and muggle (whether unknowingly or not) is generally weaker than the child of two wizards as the foetus draws magic from both parents and, due to the lack of magic in one parent, only has half the magic of a wizard-borns. This is also why a child's magic is in flux as the child is slowly assimilating and converting the parents magic to fit their unique core and reproduce more.
My brain started spewing magical theory and I couldn't fit a story into it somehow so I'm posting it. If it decides to spew a story too, I'll probably link it and leave this here, all by it's lonesome.