We don't want the father (who likely retains some element of parental responsbility until the court case has finished) to be able to get the child's location by SARs.
In Europe children are humans and humans have rights. Children are not the property of their parents.
Battered spouses frequently run away without notifying authorities, because leaving gets them out of the situation whereas contacting authorities extends and inflames the conflict.
And, because everything is a trade off, if you pass a law which is highly protective of victims then the abuser can file a false abuse complaint against the victim, then kidnap the children and deny the victim all access to their children until the fraudulent abuse complaint is dismissed.
[0] https://en.wikipedia.org/wiki/Gay%E2%80%93straight_alliance
[1] https://globalnews.ca/news/5102167/alberta-election-gsa-rall...
[2] https://edmontonjournal.com/news/politics/ucp-education-plat...
Could even be nasty in a case where separated parents have 50/50 custody and one parent uses GDPR to get data on what another parent might be doing with their child and build a case against them.