Part time TA getting 'overtime' hours back.

[HeadbandsandFlowers]

Hi I am a part time TA supporting a child in a mainstream primary.
I have been asked to extend my hours for two days next month to go on two school trips. I usually finish at 1:30 so would be doing 2 x 2 extra hours.
I always start work 20 minutes early three days a week because I don't have time to go home between dropping DS off at school and starting work (that is my choice I don't expect to get that time back or be paid for it) and I usually leave 20 minutes after I should as I have things to sort out that I haven't had time to do in the day (again my choice)
When I was asked to do these extra hours I was told I would get the time back. My DS is going on his school trip the following week so I asked if I could take my hours back then and go with him. The head said she would have to discuss this with the class teacher and SENCO and get back to me (this was last week and I haven't got an answer ) I mentioned this morning that ds' holidays don't quite match mine so if it wasn't possible to take the time back for the trip could I take the hours while he was off so it saved a day of trying to organise childcare. Again I felt like I was being unreasonable for asking for the time back and the head said 'we can't be sure that you will be doing the full four hours extra so we can't give you more hours back than you have done in overtime'
How would this be dealt with in your school? I almost feel like I am expected to do the extra hours (I also need to get someone else to pick DS up from school for those two days as I might not be back on time) and not get paid or get the time back. If I didn't do four extra hours I would obviously make the time up another time but I feel like I am asking too much?
Thank you.

Tell them you're sorry but the trips don't suit you and you will work your normal hours.
I know teachers will do extra for trips and get no time back but we are paid so much more, I don't think it's fair to ask you to do that and then quibble over the TOIL.

Perhaps your wires are crossed - who suggested that you would get the time back rather than just being paid for overtime?

I understand they don't want to definitely agree to a specific day off at the moment since you haven't yet earned the toil. However, it does sound as though they are trying to make it hard for you to take the time, maybe hoping you'll give them the time for free. In my school, we can only book toil once we've actually worked the extra time. However, they are generally good about giving us the time as long as it doesn't clash with anything important in school.
As far as the other, extra time you give them, is concerned, as you say, that's your choice. Is there any way you can do whatever you do after school, the following morning? That would give you back 20 minutes a day there. I know how it is. I arrive and start work about 30 minutes early each morning, and often don't leave on time. In the past I pretty much have them an extra 8 hours a week. And the ht at that time came to expect it and I felt guilty for leaving on time. So I stopped doing it. Our current ht does appreciate any extra time we give and always ensures we either get paid, or toil for extra time they request. And we get cakes or other treats in the staff room, at random times, with a thank you for our efforts.
If I were you, maybe the extra time in the morning would be spent in the local Starbucks. And I'd start going home on time. Sounds like they're taking your generosity for granted. So stop doing it and let them realise just what you do give.

In my school you would be allowed to go on the trip with your son. He