The grand gesture of an engagement proves nothing to the children- just catches them up in her whirlwind. Him quietly and consistently being around, them seeing their mum comfortable and happy with him, his kids seeing their dad happy and comfortable with this new woman, them learning to be comfortable with each other in all the many combinations you can end up with in a blended family that aren't always under the same roof... Actions speak louder than words and those things will show the kids permanence and reliability, not a blingy bit of jewellery and endless word salad. The engagement is 100% for them to prove they are botj still lovable and desirable in their 40-something post-divorce lives.