Hello Northern Girl,
Your teaching situation sounds really fun; I hope you’re enjoying it.
In answer to your question, I advise against teaching a child to write in a foreign language before they can write in their own language. It can be very confusing for a small child and the foreign language (English in this case) is the lower priority. If you have any doubts and you think your student might be able to cope with learning to write earlier, you should definitely speak to the parents and the child’s mainstream teacher first. But I’ll just repeat that on the whole, I advise against this.
As for progressing to the Junior A book, there is another option open to you and that is to continue teaching her with another book that reviews what she did in her first year and that doesn’t involve too much writing.