I'm considering purchasing this for my kid too. Well as far as I know, we need 350 star coin to progress in a specific part of the game and this is the initial part, it is only because this specific quest requires to have a pony which would be totally natural for any player because surely one day they would want a pony, seeing my kid playing (at level 9 currently and done so much in the game not even having the pony!) I noticed that they don't need to force it at all, if they want or need to wait until finally buy a pony, they can.
I read about it a bit and regardless of which subscription you pay for, the players will get 100 star coins once a week (being a star rider, of course), every Saturday I think. I don't know about you or your daughter and how long she will be playing but if you are like me you would analyse well and if she will be playing for more than 6 months than the lifetime package is the better option, go for it. I know it may be expensive at first but I'm saying that because if you do the math, paying by month or 3 months it is way more expensive when you think about it, I mean, the lifetime you pay once and never again! I did the counts of it and our kids can enjoy the game completely with no need to pay for star coins at all, I myself would not do that. Let's put the numbers here:
With lifetime you pay once and never again, the kid will get 100 once a week every Saturday.
100 x 4 (weeks) = 400 star coins for month
400 x 12 (year) = 4800 star coins for year
Not to mention that you can teach your daughter to search for available codes (those codes are legit) that can get them clothes and star coins for free, in the Instagram page they can see it, also there is one specific site for this matter: https://jorvikipedia.fandom.com/wiki/Redeem_Codes
Whenever a code is available they can copy it and paste in the star stable official site: https://www.starstable.com/pt/myaccount/redeem_code
Just make sure to teach your daughter to maintain the safety of her account, not sharing her credentials with 'friends' that may be there to steal her account or mess with it.
I want to buy it for my kid too, the lifetime. I am just waiting for a good promotion and I would recommend you to do the same, they have promotions every weekend so we just need to keep an eye there.
But yeah, don't worry, we don't need to buy star coins at all.