In order for the childrens' visas to be granted, you need to prove that you have 'sole responsibility' (not necessarily sole custody) for them.
The easiest way to do this is if you either have a court order giving you sole custody, or a letter from the other parent(s) confirming that you are responsible for the childrens' upbringing and giving you permission to take their children to the UK.
If you don't have either a court order or a letter from the other parent(s), you would have to find other evidence to prove that you are their primary caregiver and that you are responsible for their upbringing.
UKBA will consider the following when processing the childrens' visas (
http://www.ukba.homeoffice.gov.uk/policyandlaw/guidance/ecg/set/set7/#header8):
SET7.8 What is sole responsibility?
A sponsoring parent (see SET7.2) must be able to show that he/she has been solely responsible for exercising parental care over the child for a substantial period.
If the sponsoring parent and child are separated, the child will normally be expected to have been in the care of the sponsoring parent's relatives rather than the relatives of the other parent. An application should normally be refused if the child has been in the care of the other parent's relatives and the other parent lives nearby and takes an active interest in the child's welfare.
The following factors should be considered in assessing sole responsibility:
- Are the parents married / in a civil partnership?
- If the parents' marriage / civil partnership is dissolved, which parent was awarded legal custody, which includes assumption of responsibility for the child?
- Where there is a custody order the ECO should take care to ensure that the issue of a settlement entry clearance to the child will not contravene the terms of the custody order. See list for countries whose custody orders can be recognised as valid in UK (copy can be downloaded under 'Related documents' on the right side of this page).
- Does the marriage / civil partnership subsist, but the parents do not live together?
- If the sponsoring parent migrated to the UK, how long has the sponsoring parent been separated from the child?
- If the sponsoring parent migrated to the UK, what were the arrangements for the care of the child before and after the sponsoring parent migrated?
- If the sponsoring parent migrated to the UK, what has been / what is the sponsoring parent's relationship with the child?
- Has the sponsoring parent consistently supported the child, either by:
direct personal care; or by regular and substantial financial remittances?
- By whom, and in what proportions, is the cost of the child's maintenance borne?
- Who takes the important decisions about the child's upbringing, for example where the child lives, the choice of school, religious practice etc?
Also, is it better to be married and apply for Spousal visa, or have them all come over on the Fiance visa - would we then have to change all 3 of their visa categories after marriage or would the boys be ok and we just change my partners category?
Well, doing the spousal visa first will save you the cost of 3 extra visas (for you and your kids) after marrying in the UK (£1,800 if you apply for the next visas by post or £3,000 if you apply for them in person), plus it will allow you to work in the UK immediately.
The fiance visa does not allow any work until after you are married and have received the next visa (and if you apply by post it can take a couple of months for the next visas to be processed).
So, from that perspective, doing the spousal visa would probably be easier and cheaper.