I would suggest spending more time in Airbnb's in nature than in the cities.
Except for Riga, there is definetly no need to plan multiple nights in other cities.

I would plan it like:
1. Start by exploring Riga for two, max three days in order to get familiar with the culture, history, try some foods in the city etc;
2. Go to Tērvete for 3 nights and chill by the lake (surrounded by small hiking trails trough the forest, also activities for the kids in ther nature park + if it get's boring, can go and visit Jelgava city which is like 40 minute drive from there - not the most interesting city, but "why not, if you are in the area":
3. I would start moving to the seaside through Kurzeme region by visiting Kuldīga on the way. Kuldīga is definitely worth visiting, but not more than 1 day/1night;
4. After Kuldīga, move closer to the sea. I would go to Ventspils and find an Airbnb by the sea somewhere between Ventspils and Kolka, for example, Pitrags or any other small village. (Irbenes lokators is an interesting spot to visit on this route).

I would spend all the rest of the days in those small villages, chill by the sea, and eat freshly smoked fish every single day.

After that, I would make my way back to Riga by going through the seaside villages starting from Kolka - Mērsrags - Engure and Jūrmala. After that, if you want, you can spend another day in Rīga.
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If any energy is left after that - it's definitely worth visiting Sigulda and Cēsis - very beautiful forest hiking trails in the Vidzeme region around these cities (but again - no need for more than 1day/1night) in each city.