Oh dear. I am a retired Health Visitor, and one of my former colleagues is working in Denmark, bringing up her children bi-lingually.
I find this a little strange, and certainly hope that no-one thinks this would happen in the UK, where Health Visitors have a great deal of experience in assessing the development of multi-lingual children. So strange that I wonder if the worker is actually the professional, qualified equivalent of our own health visitors?
I certainly think that your DFD & her husband need to speak to this worker's manager and sort this out. She should be able to be allocated another worker and this one given training. I think that they especially need to do this so that no concerns are missed because of the focus on spoken language (something HVs in the UK are very alert to)
I would add that in my experience of assessing language development in multi-lingual children, there is a period of time (about 2-3) when expressed language can be somewhat 'behind' monolingual children. These are the pointers to consider:
Is the child's hearing OK?
is the child's receptive language (understanding) OK?
Is the child eating, chewing etc. OK?
Are there any other developmental concerns?
Multi-lingual children of this age often 'miss a beat' when responding, as if they are 'putting their brain' into the right language! Many people think it helpful, at this stage, to have a clear routine of what language is spoken when. This can be:
'one mouth, one language'
'English at home, Danish for visitors and when out'.
Some genuinely multi-lingual families have different days or times of day. I knew one who spoke Norwegian in the car, and another spoke French at meal times.
It just helps the little ones to know where they are.
Broad rule is that by about 4-5 they should be up to average development in whatever their main languages are.
Hope this helps