If the list of neglect issues is present, then go to social services with your allegation. Ask them to investigate. Accept their findings.
But this is incorrect:
how ever it is put across giving a 5 and 7 year old coke and sweets on a regular basis is not good and will inevitably lead to decay
There is nothing inevitable about it. It depends on the teeth. A lot of it is down to genes. Children with the same diet in any given community can have very different dental outcomes. Children's dental outcomes can vary from LA to LA. It can even happen within families. This has been pointed out upthread with examples from posters' families.
I agree with Naty's point about younger children's diet and what older school-going children find, out in the wider world. School food is notorious for hidden salt and sugar.
OP, you are very prone to making categorical statements that will be picked apart by anyone your ex partner has representing her if you choose to take her to court. You will make a fool of yourself if you approach a court with your very see-through agenda here, if all you have to go on is the state of their teeth.
Go through the proper channels here (SS) if you think there are valid concerns about the overall welfare of your children. Take it from there. If they say all is fine then you will really have to accept that and butt out of your ex partner's business.
Preferably, and if your concerns realistically fall short of anything SS would be interested in (be honest with yourself about this) I recommend you suggest family mediation to your ex-partner.
There are professional services available that are a far better choice of approach than you scheduling one-to-one meetings and trying to lay down the law to another adult and a third party, the granny.
www.nfm.org.uk/mediation-services/parents
500 locations across England and Wales.
This particular page shows the area you and exP may benefit from - post divorce/separation, co-parenting.
Do not make arrangements for mediation without first consulting your exP and asking her if she would be willing to take part in order to iron out issues that have arisen, and assuring her that you are not holding over her the threat of taking the children from her via court action.
That threat needs to be shelved. It is considered to be an extreme form of coercion for one parent to hold this over the other. Attempts to coerce have no place in any relationship.