The pamphlet is/was real; the claim "a guide to welfare" is false
The pamphlet is/was real and caused a stir in the Swedish news, for example: Aftonbladet (in Swedish)
The pamphlet was published on Socialstyrelsens website on this link (in Swedish), but was quickly retracted (in Swedish) when the criticism poured in. The main objections were that the language was too soft and not conveying harsh condemnation.
The pamphlet was not however "a guide for men who've married a child on how to use the Swedish welfare system". The claim is false because Sweden has no welfare for being married at all in Sweden, not even for people that are married legally. And in order to get welfare for having a child you must be the legal guardian or trustee of the child, which the pamphlet states clearly is not applicable in cases where someone married a child outside of Sweden.
Edit after obtaining a copy of the pamphlet
Translation by jkej from the post below (thank you very jkej much for letting me use that; readers, please give the post +1's). I have made some additional some edits as well. The order is: page above, corresponding translation below.

Information to you who are married to a child

Child marriage is illegal in Sweden
In Sweden it is illegal to marry anyone under the age of 18.
There are many reasons for this.
Children have a right to be children and to not have the
responsibilities that come with a marriage. Children should go to
school, educate themselves, develop as their own person, develop their own
interests. Early marriage often lead to early parenthood, which leads
to increased risks for the child, both in the short term and in the
long term. Getting married early can also lead to both
physical and psychological ill-health, an increased risk of
living in poverty, or to be subject to oppression and violence.
This applies in Sweden:
- All children, girls and boys, have the same rights.
- Anyone who has entered marriage has the right to get a divorce, even if the other partner does not want to get a divorce.
- Sex is voluntary, also within marriage.
- Everyone shall have access to information about sexual and reproductive health and rights.
- A girl/woman has the right to an abortion if she wishes to end a pregnancy.
- When a person has reached the age of 18 that person has the right to self-determination.
Legal guardian, trustee, or specially appointed legal guardian has responsibility for the child
Children who come to Sweden without a legal guardian are treated as
unaccompanied refugee children. If the parents of the child are here
in Sweden, they are the legal guardians of the child. Unaccompanied
refugee children are appointed a trustee who helps the child with
issues that parents otherwise help with.
If you are married to a child you can never assume the parents'

or a trustee's responsibilities. You as the husband/wife of a child
also cannot speak on behalf of the child.
Illegal to have sex with children under the age of 15
In Sweden it is illegal to have sex with someone under the age of 15.
This applies even if you are married and even if you have children
together. Children under 15 have an absolute right to be protected
from sexual activities. Someone who has sexual intercourse or engages
in another type of sexual activity with a child is committing a sexual
crime.
The purpose of these rules is to protect the child.
If the one you are married to is an unaccompanied refugee child
Social services have the de facto responsibility for all
unaccompanied refugee children. This includes arranging accommodation
for the child. It is the responsibility of the Swedish Migration
Agency to arrange accommodation for adult asylum seekers. Social
services investigate the child's need for protection and support and
suggests an accommodation for the child.
Social services can suggest that you do not live together for a shorter
or longer period. The purpose is to ensure that the child receives the
protection and support it is entitled to.
Since children under the age of 15 have an absolute right to
protection from sexual activity, it is inappropriate that you live
together if the child is under the age of 15. This applies even if you
have children together.
Both social services and the Swedish Migration
Agency will work to give the child the opportunity to express its
opinion regarding where to live and under what circumstances. It is
with the child and the trustee that the social services will cooperate
and that together with social services may decide for the child.

If you want more information about what applies to Sweden, you can
turn to the public servants at the Swedish Migration Agency or to
social services.
On the website New in Sweden you will find out more about the Swedish
society in several languages.