I have DD (3.5) and DS (8mo). DD has been obsessed with cuddling and playing with DS ever since he was born, and now he can play a bit more all she wants to do is play with him. It is lovely that she loves him so much and she is pretty good with him apart from the normal issues (constant invasion of his personal space, taking his toys off him for allegedly helpful purposes etc). But every now and again she does something really quite bizarrely silly and rough with him (like suddenly shake him really hard for no apparent reason). This has happened a handful of times, and DS has never been hurt.
Today, I left DS in his activity centre (one of those Leapfrog things), with DD in the room, and popped up stairs to put our coats away and get a change of clothes for DD. I heard DS crying and shouted down to DD to ask her what was going on. No answer from DD so I rushed downstairs and found that DD had wrapped her toy measuring tape round DS's neck (4 times!), and was pulling one end of it. I grabbed DD and moved her out the way, uncoiled tape from DS's neck, really shouted at DD that she had been very mean and silly and sent her to her room, and gave DS a cuddle.
When DD came back down I calmly explained why what she had done was wrong and dangerous, and she said she knew she was hurting him but just wanted to measure his head .
So anyway, what I was wondering is, was IBU to get very cross with DD (especially as she told me unprompted that she knew she was hurting him), or was it actually my fault for leaving them in the room together alone for a minute or two?