To be really angry and a bit worried that my dad tried to feed shortbread

[Luxme]

To my 6 week old daughter??

He put it to her lips and said 'go on then'. I said 'what on Earth are you doing?' ?He said 'just wiping a bit of sugar on her lips'. I tried not to go absolutely mental at him and said 'that's dangerous dad, promise me you won't do that again' he said (to my daughter) 'your mum likes sugar' I said 'yeah dad but I'm 35 not 6 weeks old'. He just ignored me and carried on jigging her and ate the biscuit. I reiterated to him that she's not designed to eat solids yet and he just kind of well, pretended not to hear me.

I felt like going mad at him. I didn't as so to not cause a scene. I'm annoyed because I just didn't think he'd be so stupid and secondly that he didn't seem to care.

Yanbu as it's how he handled it which is the issue. If it had been a momentary blank which he was then shame faced about then no problem. But it sounds like power play and like you get this a lot.

You need to decide your boundaries and protect them and sod their reaction. If you don't feel you can trust them with her then don't and tell them why. They need to show you they can be trusted you are under no obligation to leave her with them at all. And your mum is enabling his behaviour. If she's not willing to defend your dd's best interests in favour of pacifying a grown man she isn't trustworthy to look after her either.

Be clear now that she's going to a nursery/childminder and if they say 'we thought we'd look after her' say 'that's a nice often but she's going to x' and don't back down.

And every time he does this take her off him and pull him up on it. Ridiculous to have to but he needs to know this is a power play he won't 'win'

If your Dad reacts unreasonably if you try our something along the lines I suggested try not to lose it with him-try "broken record" -sorry your not happy with that Dad but I don't want you to give her food to taste-she's too young and its dangerous-and repeat.
Even if he gets stroppy he'll take it on board if you persist. (I have experience of awkward bastards and didn't study assertiveness skills for nothing)