To obtain a visit visa she basically needs to satisfy three criteria:-
1) This is a genuine visit for a genuine purpose.
You should write a sponsors letter outlining the history of your relationship: where and when you met, how many times you have visited her in Thailand, how often you keep in touch with evidence such as phone bills and e-mail headers, the reason for her visit at this time and what your future plans are together. This will also help with point 3.
Although given the short period of time you have known her, this may not carry much weight, but your visit in April; will help; see below.
2) She will be accommodated and supported in the UK without working or claiming public funds.
If she is staying with you, then you should provide evidence that you can provide such accommodation; either evidence that you own, such as a mortgage statement, or if you rent a letter from your landlord confirming that she can stay in the property. If she will be staying with someone else, e.g. your family, then they will need to provide such evidence as well as a letter inviting her to stay.
Finances for the visit can come from her own resources, those of a sponsor or a combination of both. Whoever is providing the finances needs to provide bank statements and pay slips (at least for the last 3 months) to show that the funds are available. There is no need to put money into her account as the funds can come from you. Indeed, sudden, large, unexplained deposits into an applicant's bank can cause suspicions in the Entry Clearance Officer's mind!
3) She will leave the UK when, or before, her visa expires (reason to return).
A job will help, and if she has one then she should get a letter from her employer confirming that she has been granted leave and her job will be kept open for her. However, having a job is not a prerequisite to obtaining a UK visit visa. Indeed, if an applicant supplies an employers letter saying they are being given 6 months holiday it is extremely unlikely that the ECO will believe it!
Having a child will not be counted as a reason to return. Many Thais leave their children with relatives while they travel long distances, often abroad, for work.
Many couples have obtained a visit visa when the Thai partner has not had a concrete reason to return based purely on the strength of the relationship. If the ECO feels that the relationship is long term and may lead to marriage then s/he will feel that the applicant would not want to jeapordise any future settlement application by overstaying or otherwise breaching the conditions of a visit visa.
However, I don't really see this happening on the strength of a 4 week relationship, although the time you spend with her in April will help. During that visit, gather as much evidence as you can of your time together, e.g. photos of you together. Also start keeping your phone and e-mail etc. records to show how often you contact each other while apart.
If her child is coming too, then he will need to submit a separate application and pay the same fee. Where is the boy's father? Unless your girlfriend has legal sole custody, or the father is dead, and can prove it then you will need a letter of consent from the father. See
Special Visitor: Child visitors.
There is no minimum time limit between visit visa applications. However, visitors can usually only spend a maximum of 6 months in the UK on any one visit and a maximum of 6 months out of any 12 in total in the UK. Any application made where it appears these limits would be exceeded will probably be refused.
See also
Guidance - Visitors (INF 2) and
Guidance - Sponsors (INF 3).
IMHO it is far too early to start thinking about marriage and settlement. When the time comes post again and we can then give you advice on settlement.
