A baptism is a special service that symbolises that the one being baptised is welcomed and belongs to the family of the church as a follower of Jesus. Often it is an infant being baptised. During the service parents of infants make promises to live within the Christian faith and to support their child to grow within that faith. Members of the church also make promises to support the family.
Baptism, however, is open to all ages; instead of parents making promises on behalf of their child, the one to be baptised will make promises to live within the Christian faith and, again, the members of the church will make promises to support them grow in faith.
For some parents, and for a variety of reasons, a baptism service may not feel to be the most appropriate; however, they want to give thanks to God for their child. In these instances, a service of thanksgiving may be a more comfortable option. If you are unsure, talk it over with the minister.
The following links take you to more information from the Methodist Church.