Dd1 kept doing this, and there was a lot of blood. She hated having to hold the tissues so we tell her stop or you will make blood. I also took her to our gp and he told her not to play with her nose, he also did this when she was putting sultanas up her nose. She loves him as he is great with kids and she has listened as she hasn't made her nose bleed since. She does still pick to 'get the boogers out' as she tells me.