eldest is 19m so 2 young to understand naughty seat but old enough to know ive told her off for doing wrong....and cheeky enough to carry on being nasty to her 6m baby sister. thowing toys at her head, slapping and she pinched her cheek hard enough to cut the other day....i try to equally share my time between them, occupy them and let big sis help me with little sis.....but still shes feeling jelous and it comes out as hurting her. help?